I would go the GT40 single route without question if I was starting afresh, but there's the expense of turbo, manifold, downpipe, inlet, oil and water lines etc. to consider; whereas I just need some new cores and maybe exhaust housings to get going with what I have.
It seems that the 0.61 exhaust housings might have been too small in any case (RB26 kit usually comes with 0.87 A/R) - only just over 500 at the hubs at 1.6 bar on a Tomei 2.8 bottom end and A'pexi V-Max 270/11.4 cams ...
Phil