How much power should i be making once meth injected??

[The Godfather]

ok well basically weather i use C16 or E85 or water/meth im going to have to add more boost and timing to see any hp gains

so my initial question was roughly how much gains should i see??

say with my current setup im on 270awkw at 20psi at 7800rpm and if on water/meth i went 28psi @ 9000rpm what would be my rough gains if i followed richardL's guidlines of:

100% water..... run 10-15% of water to fuel (300cc to 450cc/min)
M50:W50 ....... 15-20% to fuel (450-600cc/min)
M100% ....... 20-25% to fuel: 600cc to 750cc/min

what are the downsides to running 100% meth injection? is it corrosive?