Well, I wouldn't expect to see any big difference in performance to be honest if you swapped a set of good injectors for another with the same flow rate but better spary pattern. There might be a tiny reduction in consumption and emissions but I doubt it'll make any more power as there are no other mods. I kind of assumed QF had a reason other than performance for doing this - maybe there is a problem with the older injectors?
The only way a change in injector is going to do anything significant for power is if you are running at the limit of what a certain set can provide and you are having to limit boost as a consequence. If a nat asp. motor is running short of injector flow you really are in trouble! All you could do there I suppose is limit revs until you can sort it out.
So, for a nat asp. twinny it would not be worth doing (unless the original injectors were FUBARed)....in which case any injector with the same spec as the originals would do fine to restore the original performance.
Main thing to remember is that the engine only needs as much fuel as it can use...and this is determined by how much air you can get into it. Pouring more fuel in won't do a whole lot for power. Check out the comments about the e-bay special power boost resistors in the back of this months PPC mag!...and check out next months issue as Dave Walker is starting a new series about engine managment. Now there is a real expert with masses of real-world experience.
An example. A car I was involved with recently - twin turbo 12V V6 164 motor. At 12psi of boost, it ran a maximum duty cycle on the injectors of IIRC 92%. Pretty much all the experts will say that much over 85% and you need to look at changing for bigger injectors or upping the fuel pressure. Later development on this motor will mean some bigger injectors than it has now, and ECU boost control. At the moment, it's running simple actuator set boost limiting. Somewhat by chance, it turned out to be almost perfectly 'matched' in terms of boost and available fuel flow. It would have been quite feasible to have had more boost than the injectors could have supplied fuel for (which would have been a problem), or more fuel than it could have used, had the injectors been bigger (which wouldn't be an issue as the mapping would just have opened them for a shorter period).
But don't forget what I said, I'm no expert!! This is just stuff I have picked up from the real experts and I think I have got it straight in my head