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Basic question:

Will 8Gb vs 16Gb on the rMBP have any affect on the lagging problem?

Absolutely none.

Unless you're running a ton of applications and your system is sweating to survive on 8GB, but in that case the lag would be from an entirely different cause.
 
THough having 16gig will increase the vram allotment in the hd4000 compared to 8gigs from 384mb to 500mb iirc

Is it possible to manually do this - as I feel i only really need 4GB ram anyways, so i could spare more for the video card.
 
If you have been actually following the threads you will see that the release build on ML 10.8 cures the lagging issues. This has always been a software not hardware issue. And I would only buy the 8gb machine if I plan to only keep it about 2 1/2 years honestly. The ram upgrade is only 200 bucks iirc.
 
The lagging has a couple sources causing it, but the main one is that the UI elements that are lagging can only use a single core of the processor. They've addressed these issues in Mountain Lion.
 
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