BSA were not known to shout too much about this type of thing, and you probably know you will get a lot of differing opinion on this, but this is my view.
Your 40bhp @ 6000 seems about right for an ally head bike, Flashes started out at around 35bhp but that increased as they were developed, particularly with the DA10 engine which had the same bottom end and 356 camshaft as the Rockets. The 357 cam engines probably made a few more than 40bhp in the higher rev ranges.
The question is, would you push an un-known engine to 6000+ rpm, personnally I wouldn't, but a properly built engine should be ok in short bursts. It's a question of performance over longevity. There are some great modifications to help with this, SRM make a great oil pump and those MAP conrods are a good mod for sustained high speed running.
The thing to remember is that these engines were designed in the days before motorways / freeways so engines wont tollerate 6000 rpm all day long, but they will be happy cruising at legal speeds.
Enjoy.
