As you might have suspected, the indirect comments to your question are a reaction to the non-Pcar powerplant now pushing your 912.
I have a feeling you are (or will be) lacking in the whoa department. Not sure why the PO upgraded only the rear brakes. 912's (the early ones) originally came with solid rotors with M calipers. These brakes are better suited to 911s without Chevy V8's in them. The front brakes are more important in slowing you down, especially with the +300 hp bowtie engine back there. If your front struts are stock, you'll need some late model struts (3.5" caliper mouting bolt pattern), then seriously look at the myriad of brake upgrades available which will all bolt up to these struts. Your brake upgrades will dictate minimum wheel size as well. Do a search on this site and you'll find a wealth of info on any combination of brake/rotor and caliper upgrade you can imagine.
I'd also suggest performing a similar search for suspension upgrades.
Regards,
Sherwood Lee
http://members.rennlist.org/911pcars