I've seen over 80mph on a slighly modified TC5. Trust me, that is faster than it seems. There is so much power that you can break the wheels loose almost at will up to say 50-60 mph. Even small steering corrections can be nerve wracking at that speed - obviously you'll want a transmitter where you can configure a setting for top speed stuff - as you'll want completely different settings than a normal bashing or TC racing type setup.
Small bumps @ 80mph on a 1/10th touring car are quite exciting. You start to worry about things like gusts of wind, radio range, and enough room to shut the thing down w/o causing any damage.
Here's video of a guy's TC5 experiencing a very entertaining blowover at about 100 mph. If $'s are a concern, 100 mph blowovers are not going to be enjoyable though.
Also, if $'s are a concern, you should be aware that the faster the thing is going, the more things you're going to break when it crashes (it happens to the best of us) or just from general stress.
For some perspective on that speed:
Spektrum 2.4 GHz radios have a roughly 300' range, so that's 600' of radio range (unmodified) using max enpoints. I find that with that radio, I'm at least as limited by radio range and braking as I am by engine performance acceleration and speed-wise.
Accelerating to that speed, while under control, on any "normal" surface, quickly is not as easy as you might think. At 80 mph you're doing 117 feet per second. That's about 4 seconds of acceleration and speed before transitioning to shutdown. I find that the car is still capable of further acceleration when I have to transition to slowing down, or I risk something "bad" happening. The real go-fast guys have a lot more runway (both radio and pavement) than I have easy access to.
It's to the point where I ordered a 3pk with the FASST module as the true "go fast" pros on http://www.fast-rc.co.uk/forum/index.php
have convinced me that even though no one can technically explain why it is (its voodoo essentially) the range on that particular transmitter is about 3x that of the Spektrum.
With a 5700kv on 3S Lipo, geared right I think you'll find the speed is at least a very good place to start. If you need more after that, on 3S the Castle 7700kv would be faster still. While it isn't "supported" it'll work at least some of the time.
I would guess that with a fast motor, sufficiently tall gearing, sufficient radio range, and a nice smooth stretch of asphalt, 3S Lipo would break 100mph.
Here is the same thing on 4S Lipo @ 122mph
Here is the same thing on 5S Lipo @ 135mph
At speeds beyond 100mph, crazier voltages, more aerodynamic bodies, tire reliabilty, etc all start to play issues. Drag is a bitch.
Once I get a longer range radio, hopefully I'll be able to tell you what it took to break 100 mph from personal experience. Heck, some of the project cars designed to go really
fast have frickin rudders as steering. (or at least partial/high speed steering)
FWIW, top speed setups and drag setups are different. Drag setups are designed to cover a set distance in as short a time as possible. They're all about the launch and acceleration, so lower gearing, lighter weight, hooking up, etc (and keeping it under control) Top speed setups are less about acceleration, and more about stability, tall gearing/rollout and voltage. (well, and aerodynamics, etc) Another thing to keep in mind, is that not all kv ratings are created equal. Ultimately if you can gear properly (and rollout properly), wattage (or horsepower) would be a better motor indicator I think.
Now, with all those disclaimers and warnings, it is also completely true, that there's something sickly fascinating about seeing how fast you can get one of these little electric monsters to go.
I've shown 70+ mph to some people that aren't even that into rc cars, and they're like Holy Crap WTF was that!?!?!