That all depends on how the head is machined. With a standard head I would think you get too big squish and improper burn of the fuel / difficult jetting. Did you measure the squish?
No, like an fool I didn't. It was snowing outside and I was in a hurry to finish putting it all back together so I could go rip in the snow.
I used the same thickness base gasket at what was originally on the bike when the the PO had the squish mod done. So I figured "it would be the same" now I regret not measuring.