First thing to bare in mind is you aren't actually gaining anything - the power is already there, this just a similar to transmission loss. Under-run pulleys are a bit of gimmick and often don't 'reclaim' the power they claim to. I'm not sure you'd want to start under-running the alternator, especially if you have a lot of ICE or a smaller light weight battery.
If could see hard dyno-proven evidence on a real car and not Neuspeed's super-duper-magic 'CAD' generated figures then I might be convinced otherwise but I think I'd give this one a miss. You're not going to notice any real noticeable difference in engine pick up or power on the road. With turbo engines it's all about the torque, which does the actual work. Under-run pulleys are probably better suited to N/A engines where every little helps and there is less torque to over come parasitic drag from alternators, various pumps and other engine driven ancillaries.
