Garrett, with a stock HX-35, you will see more lag. The HY-35 is pretty nice in that regard.
Now with Piers' 35 and a 12cm housing, lag is less than a stock HX-35, and very close to a HY-35. Again, this is dependent on where you live. The turbo obviously acts different geographically.
I know that when I went up to Canada from Arizona, once I hit Oregon and as I went further North, the lag felt even less. Again ,this was in July, outside temps went from 110+ to 70.
If you're not going too crazy on mods, no reason to change the HY-35.