You've perhaps just answered your own question. Custom engine upgrades will likely cost more than you may be willing to spend as custom parts would need to be made, etc. Plus there may be reduction in reliability when increasing rev limit & valve lift. It costs big money to get high power from these engines and things will probably break in the course of using them.
These bikes aren't about making top power, nor will they be competitive against newer more powerful factory bikes on the track in a straight line. They're about handling, feel and character.
Spend your money on quality suspension upgrades and lightweight wheels, which will pay better dividends than chasing higher power figures. Just my $0.02 worth.
Euro RC51 SP2: HRC WSB Ti Hi-Level, T1 Airbox & Snorkel, PCIII, Dymag CA5 Carbon Wheels, Ti64 Spindles (F, R & Swing Arm), Mori Link, Maxton GP7 Shock & Ohlins 832 Forks, Brembo GP Calipers & SBK narrow track (F), Braketech Disc (R), Brembo billet Underslung x 2 Brembo RCS, Tyga Triple, 7075 Stem, Probolt Titanium all over, Babyface rearsets, MR Complete Carbon Lineup, Custom Undertail & CBR 1000 LED, Watsen LED Indicators, Harris Brace, Giles clip-ons, Corbin seat, etc. 2013 BOTY.