I think it goes - bigger bore = more top end horsepower
Narrower bore = more mid range horsepower
Bigger bore also equals more noise - have a look at pics of Chillis pipes on his 998.5 - big pipes, smaller outlets to keep within noise regs
I run 45 - 50mm on my 996 - made 127.6bhp on the Moto Rapido dyno
My 916 with same exhaust made 106.8bhp on the Moto Rapido Dyno, so from Skidlids figures the extra bore looks like it's worth 2 or 3 horses - although other factors in the engine and transmission would easily affect these figures
I think there's a bit about this on Doug Lofgrens site
http://www.visi.com/~moperfserv/916comp.htm [Edited on 10-12-2005 by Webcore]