ORIGINAL: The Mad Modder
As I mentioned in post #9, my brother's vintage blitzer beetle is currently running a Tekin 10.5 brushless system with no troubles. So, yes they will handle more than the stock 6 cell w/silver can motor setup. Many of Tamiya's cars and trucks(especially the blackfoot xtreme and dual hunter) have overbuilt gearboxes with wide gears that are made to take a good deal of abuse.
An RCUer here (Lunchboxer) runs BL stuff in his Tamiyas a decent amount. I believe he has a Blitzer Beetle he put a fast BL setup in and took video up, the gears hold up fine. He also ran a really quick BL setup in his LunchBox w/out issues (other than handling...haha). If the plastic gears are stout enough (and Tamiya's are), they can handle BL. Just because the kits come with a 540 silvercan doesn't mean they can't handle anything else.
Tamiya makes quality stuff. They just don't shove "metal gear" advertising down our throats. [&:]
I have a Stadium Thunder (same guts as the Beetle here) that runs 2s LiPo on a 14T Titan 550. I know that's not BL power, but I abuse the snot out of it, and the gears are the least of my worries. Thinking about it, I've got a 550 can in there with no slipper, no extra gear protection, and it's solid.