Erm, its a mathematical formula mate, not a guestimate.
HP = LBFT * RPM / 5252
Thats physics. If you have either of HP, LBFT for any given RPM you can calculate the other value.
Take your power plot above and run the numbers and you'll see what i mean. 3500rpm for example its making 360lbft. if we do 360 * 3500 /5252 we get 239.9, which matches what the graph shows.
I dont see why restricting the torque would require overfuelling or leaking boost?
You control the boost level, with the N75 valve just like normal, to remove the torque spikes.
Like this:
http://www.jesperdk.com/etomic/powercurve.jpeg
Audi doesnt produce that by overfuelling, they control the boost level, to produce the desired torque map. If they went full boost at 2000rpm, you'd get a huge spike. Instead they peg the torque at 550nm (in that example) to protect the transmission.
Proper mapping can produce a result like that with any turbocharged engine, by limiting the boost at lower revs, then ramping it up as the VE drops as the revs increase.
The reason a 2871r doesnt produce the same dangerous torque spikes, is because it spools further up the rev range.
I'm not disagreeing with you on the stock internals. I'm agreeing, and saying that even with a torque limit of 550lbft, if you have a well crafted map and a turbo capable of it, you can make 700+ hp.
A mate of mine is building a Volvo T5 engine for his locost. The torque limit on those is around 300lbft and the rods are a particular weak point. Hes aiming for 300lbft from when the engine comes on boost at 3000rpm until the rev limit of 7000, giving him 400hp. Hes specced up a 400hp turbo, and will be using some mappable engine management to control the boost using something like an N75 valve, with low boost coming in at 3000rpm, and scaling up as the revs climb to keep the motor producing 300lbft all the way thru the revs.
