Roy and I are both making 260-280rwkw on 1.2-1.4 bar with TD06 L2's, and I get full boost before 4,500, around 4,200 with a slight manifold leak at the time (that grew bigger).
And I don't run out of puff at limiter 7,600 rpm (but I have a different intake manifold)
I am thinking, unless you want well over 300rwkw (say 350+) don't bother. But by that power level RB25's make sense