I have the new MBP (with the M370X) and haven't noticed any lag/stutter issues, even when using the Iris. My only real complaint with this machine is that the fans are noisier than the 2011 at idle, but I'm sure that I'll get used to that.
I've gone back to default settings for the display and am now using webkit nightly. At least in those aspects there is some improvement now.
The applications folder still lags/stutters and yes, Mission Control still doesn't play too nicely but oh well. Now I am just waiting for 10.11 to fix the mess Yosemite has been in terms of performance.