I've done ~ 40 mph downhill, on good pavement, while riding a "decent for the time" MTB. I found this to be more "eye opening" than doing just over a buck fifty (also downhill on good pavement) while topping-out a Yamaha FJ1100 (had about 17 years of motorcycle experience at the time). The thing was that I could actually stop the Yamaha. However, I had foolishly "over-ridden" the canti brakes on my MTB and, as such, I couldn't stop it (until the road leveled out some). I can also tell you that this was one twitchy MTB ride. Yep, lots of fun.
Sure, my MTB didn't have proper road type tires (PSI was maxed tho). And no, I didn't expect a MTB to handle well while traveling that "fast" on pavement. Yet, I must have routinely reached speeds of 35 while riding the same bike on certain off-road downhill runs (that often had "berms" that helped keep you on "track"). Had some really good, and few not so good, times on those downhill dirt runs. This was, of course, more or less what it was made to do. Nonetheless, I too feel that 40 is about the maximum for safe budget sensible e-bike conversions. Now I'll admit that people can, and do, go quite a bit faster on their e-bike builds. I myself would much prefer doing so while running capable brakes, proper tires, a purpose built frame, and proper weight distribution. The problem for me is that this would cost about as much as a motorcycle that offers far superior performance.
And ya, I too have found that e-bike conversions can be very picky about gearing. Too tall, and you'll over-load/heat the power system. Too "short" and the thing becomes all but useless (you be better off with a regular pedal powered machine). Besides, it must look pretty funny when you're windmilling like crazy on an under-geared e-bike. Luckily I've been flying electric RC fixed and rotary winged aircraft for about 6 years and, as such, realize that it can take a lot of time and considerable trial and error when it comes to configuring electric power systems.
Wheelbase is, indeed, probably one of the biggest safety hurdles with high speed electric conversions imo. The Worksman framed BT racer I'm working on has horizontal rear dropouts that allow for a wheelbase of 42 to just over 44 inches (my other MTBs and road bikes run ~ a 40" to 41" WB). I'm currently @ 43" which gives more than reasonable stability at 20 mph. Note: This single-speed BTR build has really proven to be a challenge in the gearing department (I suspect that I'll be buying more than a few different tooth-count freewheels before I find a usable ratio). The good thing is that I only ride this "BTR" on level hard-packed dirt roads, which should make the final gearing selection easier.
As for your thoughts on pedaling: the average rider on a regular bicycle produces maybe 150 watts over a period of time and what, ~ 250w for a 25 second burst? Well if so, that's some serious power to allow to go to waste imo. Those, like you and me, who subscribe to this belief might do well to consider some of the high powered geared hub-motors, or a "freewheel" type of "mid-drive" power system. IDK, maybe you've incorporated this into your superb build...