I don't think this can be measured in a vacuum.

Let's explore some different cases as a thought experiment:

A country achieves communism. I don't think this is possible tbh but let's go with it.

We can expect to see the "proles" consuming more than they would otherwise as they'd have all the products of their labour.

Let's presume that people are people and that they aren't going to suddenly develop much more class consciousness and a spirit of internationalism. Obviously I think that a communist society would go a long way towards this but let's ignore that for argument's sake.

Unequal exchange would mean that the communist society would be taking advantage of this arrangement, perhaps more than they would be able to otherwise.

But if this is the situation, we also have no bourgeoisie who do rampant exploitation of the third world. We have no more corporations. We have no more bourgeois democracy inflicting imperialism upon the world.

Perhaps consumption drops a whole lot purely by virtue of the fact that people would rather work 4 hours a day or 3 days a week. Perhaps in freeing up the products of labour and what would otherwise be capital and surplus value under the previous system, people are able to manufacture and acquire products designed for repairing with replaceable parts rather than for planned obsolescence. Perhaps people would be able to be more conscious consumers, opting for the things that have a lower environmental and social impact rather than working two jobs as part of a single-parent nuclear(ish) family and only being able to choose the simplest and most readily available options rather than carefully considering what they would genuinely prefer. Perhaps lots of people devote their time to things like gardening and producing food themselves because they only need to work 15 hours a week in their factory job to cover the rest of their needs.

It's hard to estimate what it would look like exactly, especially in an unbiased way, but even in a conservative estimate I'd say that it would be a net-benefit for the third world as the degree of exploitation and the worst excesses of consumption would be largely curbed, not to mention all of the excesses of capitalism and imperialism being eliminated (from that society anyway).

So let's look at a genuinely SocDem society next:

Imperialism is dead in the water. Capitalism is hemmed in. Billionaires are reduced to having no more than, say $10 million in net worth. If corporations still exist they are brought to heel and they are held accountable for their inevitable excesses.

Honestly in this society I would expect the net benefit to the third world to be worse than the example above but it would still be much better than what we have today.

Next is to consider things as they are today:

Increased wages are going to lead to increased consumption. But things like earlier retirement and better healthcare, education, environmental and workplace safety etc. are going to reduce the impacts on the third world - healthcare, especially stuff that is way downstream, has a big footprint. Workplace and public health and safety makes things better for everyone. Carving out chunks of profit to go towards better conditions generally means less money for wars and less money going towards imperialism, not always but more so than not. Workers having unions and solidarity means that there's more chance of things like general strikes, which can achieve good outcomes for the third world.

I think under this scenario we could expect to see a net benefit that is significantly reduced compared to a SocDem hypothetical scenario. It might even come out as a wash, if you really want to make a conservative estimate.

Idk this argument seems overly simplistic and very undialectical honestly. It's a bit like the reactionaries who complain about veganism or measures that benefit the environment and they charge vegans with being responsible for the deaths of animals due to industrialised agriculture or they concern-troll over the carbon footprint of a proposed expansion to rail transport.

I mean, yeah, there's definitely an environmental footprint that gets incurred when you manufacture a car seatbelt and that's fine. But if 100,000 seatbelts prevent one single person from becoming a permanent wheelchair user then the comparative environmental footprint is vastly in favour of making those 100,000 seatbelts because the environmental footprint incurred by the necessary medical and accessibility interventions from one preventable case of someone ending up as a permanent wheelchair user are far greater.

This is not an argument in favour of eugenics or to lay the blame for the social and environmental impacts of being disabled at the feet of the individual though. I'm just trying to highlight that we should not fall victim to an overly reductionist assessment of things in a very static way or otherwise we end up with well-intentioned measures that can have ramifications that are far worse than what we prevent.

Likewise we should not oppose fighting for better working conditions in the first world out of concern that any improvements here are simply going to make things worse in the third world because it's not nearly as simple an arrangement as one where improvements here necessarily make things worse over there in equal measure.