Do you thing an upgrade in the Intel HD 4000 VRAM up to 1576 mb would solve those lags in animations?
I remember they doubled the VRAM (from 512 mb to 1024 mb) with Mavericks.
I highly doubt it. Thing is, it isn't a lack of VRAM that's causing the stutters and lags but more likely bad software optimisations which puts more load on the HD 4000 when it's already working to the limit in past OS X versions. With more powerful graphics card like the dedicated 650m, this is less likely to occur.
At this point in time, some possible options that will minimize these problems would be to:
1. Make sure your resolution is set as "best for display" instead of the HiDPI resolutions
2. Turn on reduce transparency
3. Reduce animations (eg. in dock preferences, minimize windows using scale effect, uncheck animate opening applications, disable magnification)
4. Turn off font smoothing in general preferences (not sure if it does much but it
is GPU accelerated. Also, you shouldn't even need font smoothing on a retina display)
5. Turn off dashboard in mission control preferences (as some users have reported)
6. Hope that they release updates for software optimizations and update their graphics drivers
7. Downgrade/stay to mavericks for the time being
8. Resetting the SMC
might help (google it if you don't know how)
You can also try the following steps (Disclaimer: do not blame me if anything bad happens):
1. Deleting /Library/Preferences/com.apple.windowserver.plist & ~/Library/Preferences/ByHost/com.apple.windowserver*.plist file
2. Shut down computer
3. When turning on computer, reset PRAM (again, google if you don't know how)
*Might have to reconfigure display preferences.