There must be a point where it is physically impossible to take a brick that that and not go any faster. So what is that limit? Any ideas?

I am not quite sure that I understand the question. The top sprint speed or downhill speed on a bike is determined by how fast you can spin your legs in combination with the number of teeth on your big chain ring on your crankset. The big ring typically has much fewer teeth on a mountain bike vs a road bike so your cadence would hit a limit at a lower speed. In that regard, if you could spin say, 160 rpm you might max out around 30 mph (taking a sort of wild guess) on the mountain bike but at the same cadence on a road bike it'd be around 40 mph.

As for average speed riding a mountain bike on the road, that is going to be determined by your conditioning. Granted there is an upper limit but an upper limit but for a regular guy vs someone like Bradley Wiggins could be huge.

The tire patch size hitting the road is also a huge factor. The numbers I've seen are that riding your MTB at any given speed takes 50% more power than riding a road bike at the same speed. That number goes up as hills get involved and speed goes up and can hit 100%.

If you use Strava at all, you can log a ride with your MTB, then make a virtual road bike, and it will calculate the power you "would have" exerted if you did the same ride on that road bike. It's pretty eye-opening - MTBs are one hell of a work out.

I see where you are coming from. There is definitely a formula somewhere (inverse square law?) I think, if I recall college math, if you want to go twice as fast you have to produce 4x the amount of energy. This is why my compact car with 120 horse power will go about 120 mph but a car with 400 hp will not go 400 mph, although, it'd be cool if it did. So, relating back to the bike, you have a few factors that will effect max average speed as compared to a road bike. Increased roll resistance from the knobbies and more wind resistance than riding in the drops on a more aero frame. Also, the weight would not be much of a factor on flat roads but would be on hillier terrain.

It is cool to think about. We need someone with more geek cred than me to do some number crunching with some specifics. Say we have an average dude producing a sustained 200 watts for 30 minutes vs a pro producing a sustained 400 watts, all other factors like body weight being the same.

We need someone with more geek cred than me to do some number crunching with some specifics. S

https://strava.zendesk.com/entries/20959332-Power-Calculations

There is part of my head which says "I'm doing these speeds on a MTB, just wait until I get on a road bike"!

Or even your 15lb TT bike, that must be insane!!

There is part of my head which says "I'm doing these speeds on a MTB, just wait until I get on a road bike"!

QFT! Same here. Training harder than the race is a time honoured path to success!