I run a 3 speed and reverse in my Savage. Probably one of the better things I have ever done other than a new engine and dual disk brakes. I have had this setup on my Savage for about 3 1/2 years now with no failures because of it.
My recommendation is that if you don't want the mixer, then you need to hold the brake while engaging reverse so you don't ruin it. Then hold the brake until you throw it into forward gear. I use a standard strength servo so to ensure it engages the gear, I let it idle for a second or two to give the servo some time to catch up. So long as you remember that, you should be fine. One nice thing about the mixer is that it forces you to hold the brake in order to switch to reverse and so on. If you are going to race your Savage, you might as well spend your money else where.But ever since the onslaught of the Truggy class, who races these things nowadays?
I race mine from time to time, but it is mainly a basher and isn't my primary class. I just race it for fun against T-Maxx's and such. Its a good ol' time.
Hope this helps! By the way, your Savage should have no problems at all incorporating both the 3 speed tranny and reverse. HPI actually designed the 3 speed that way! With the 3 speed you should be looking at a slight increase of speed and torque at the wheels, but nothing too crazy. Have fun!