I have the phase 3 from Roush, I had it done at Roush Yates in Mooresville, NC. They will stand behind any work they do, mechanic gave me his personal cell, and assured me any problems at all that they will fix them. The roush warranty would stay in place for parts at least regardless with the Roush tune. People say it's conservative blah blah blah. I pulled 572 rwhp from phase 3 upgrade, it made it into a different car. You need it in my opinion! I am adding kooks LT headers with an off road H pipe 3/26, I also changed a few other things. With that they have to custom tune it etc. I will pull over 600rwhp for sure. Anyways, HP is more of a number. I'd say it's more about getting traction. The phase 3 kit from Roush would not dissapoint you, plus you would have the Roush CAI (not sure if VMP uses that or another). I like the looks of keeping mine mostly Roush anyways...with the exception of some things I had to upgrade. As for VMP and warranty....Yeah they are salesmen to some extent. It is mostly dealer specific. If they think you modded car, they "might" not be mod friendly. Some dealers have turned me away for warranty work on other cars, some not. At the end of the day, battery or not....they would know it's modded and if they feel like it void warranty. I wouldn't sweat it much, Roush is safe.
Phase 3 upgrade including CAI, Fuel pump voltage regulator, (629rwhp), 543 torque, Kooks LT headers with off road H pipe,ID1000's, Steeda Heat Exchanger, SCT tuner with custom Pro Dyno tune, BMR LCA's, BMR relocation brackets, BMR adjustable Panhard bar, BMR Rear sway bar, Steeda Aluminum driveshaft, Steeda X-brace, Steeda Differential cover, Steeda torque box brace/frame rails, Shelby trans cooler, Mcleod SS clutch line, RP fluids, whiteline transmission bushing insert and MGW race spec short shifter.