You're right, you tend to hit a limit, normally cause the turbo is off its compressor map and starting to get ineffiecent, or cause the engine CR is too high to accept any timing at that boost level, or a combination of both.

It shouldnt be an issue here though, he is only pitching at the same level of tune as a 280bhp saph, it just sounds a lot cause there is a lot of cubic inches involved!

He will be down to 8:1 CR by then as well, if he was trying for 1000bhp on the current 10:1 CR i should see your points possibly being an issue, at the very least he would be running hardly any ignition to be safe at a bar of boost on 10:1, but he wont be doing that, he wants to install it as it is, and then see if he thinks the chassis and running gear can take any more power before he rebuilds it at lower CR.

If he goes 8:1 then potentially he could get 1500bhp out of it, but he isnt after that, he is after moderate power and reliable, a set of 285 wide street legal tyres are going to struggle on 1000bhp anyway TBH