Whatever kit you get for water/meth injection, make sure that the check valve is warrantied and of good quality. If not, water/meth leaks into your intake or IC when the car is off and you get serious issues. SRTforums has lots of threads about that (SRT-4 related mostly).

Also, if you tune and your exhaust gas temps are high due to tuning or timing advance, you have to make sure that the water/meth comes on at the right times, ALL the time, or the heat build up can melt a piston or piston ring. Again, this comes from SRTforums, mostly on the SRT-4's.

EGT temps on the SRT-6 may not be as high though, but there can be a downside to water/meth injection if it's not working properly or tuned to come on correctly, especially at high RPMs and temps.