I have ben running 290+ HP on stock injectors for going 2 years now, but I do stay on top of the tune up, cleaning and lubeing the FMU twice a year, I run really high fuel pres, I also know its really on the ragged edge fuel wise, 275-280 would be safer but like I said I really keep a eye on the tune, and check plugs often, to go ANY higher HP would def need bigger injectors S2000s will get you to 300-325 maybe a bit more safely, are not to bad to tune, thing to remember is when you go bigger injectors with a single stage kit, (4 injectors) they are real tough to tune for street use, most tuners can not, some can and do a great job, but its a big pain in the ass,......... now with race only bikes the skys the limit, put in 5 or 6 hundred cc injectors dyno it, and run it, since low speed drivability is not a concern
S2000 injectors can indeed be tuned correctly for driveability and horsepower quite easily with a modified FMU and a powercommander. When it comes to 65 pound injectors, you need to be able to go into the ECU - 2002-2007 Busas - to make fuel map modifications; again, with a modified FMU, you can easily get the proper fuel delivery you need to go over 400 RWHP, and you can also get excellent low speed driveability.
Anything over that on an early 'Busa would require piggybacks, or a standalone to get right . . .