I think there are two schools of thought, IMHO.
There are those, the majority, who will buy a Ninja 300 and enjoy it, grow into it, learn, then grow out of it. That is the Majority.
There are others who just have to play. Have to experiment. There are Turbo Kits for the old Ninja 250 and they sold well. Yes, you can get a big bike with 150hp but that is a cheque book purchase.
So, if a guy wants to get a buzz from blowing a GPZ1000 away with a smaller bike, there will be those who will just want to do it, for the hell of it. I do agree that the brakes will need upgrading but that is not a difficult or expensive task. Get bigger brakes from a bigger bike. SOme may argue 'why would you bother' when you can simply get a bigger bike. There are some that just can't help them selves.
I admit I am one of these but for me, if one guy wants to buy a big bike and another wants to mod another, be it a diminutive moped or a H2 of the shelf unit, each to their own. By all means warn of the pitfalls but I would not look down on either path.
For the record, Turbo's are driven by heat, not gas flow. I have been asked if my head kit would also accept a turbo and as I have pointed out many times, the gas from the rotary valve removes far less heat from the gas charge as it exits the head, so would spool up earlier (so the theory goes) and would not be as temperamental to knock or require reduced compression (or not as much) due to the removal of the poppet valve which retains such heat.
Today the RVi head has come back to the workshop as the new rotor seals finally arrived and maybe, after I get it running, I might just pop a turbo on it too. Seeing as I've removed some complexity and weight, I could just go and add it back again.
On the integrity of the rest of the engine, yes I can see a massive hike in power might cause some mechanical reliability, but having poured over almost every cog and sprocket of this engine, I can tell you it is very, very well built and could easily take more power but there is a clutch upgrade which I would recommend.
Brakes, well yes, they are great as is but could be upgraded. Simple, look for a ZXR250 in a wreckers and fit the forks,. gaining a twin disk setup. O think they are already pretty good and better than some superbikes of a decade ago but if you add weight and power, they might need a little beefing.
To quote the Triumph sales catch phrase, some want to go their own way. For others, they are skilled in other areas that are not mechanical and so have enough cash to simply upgrade. What ever floats your boat brothers and sisters, but I would supercharge it my self, and I actually just might. There are some nice small one's about with good low end grunt available.
Again,. these are only my humble opinions and I think there will be a market for these turbo kits. Limited maybe, but someone will want to.