Your cam does have 16* of overlap and is mild for the combo. With turbos IMO mild is all you need.

Advancing the cam 4* would put it at a 109* ICL, but the exhaust closing point would be 4* ATDC not at TDC as you stated. Crank * not Cam *

Your interests are Spooling up the Turbo faster, So IMO Id get a Split duration cam that opens the exhaust valve sooner before BDC Or maybe Advance the cam you have now even more then its 4* Maybe like 8* for a 105* ICL

IMO even with boost a little bit of overlap scavenging isnt a bad thing. mike