Aye, the biturbo D5 was a peach indeed. Mine is the 185, and while it's no rocket ship it's not a slouch either. I don't know the exact specs but the Polestar map adds 8 or 9 brake and another 30or so NM of urge and the engine response between 60, when the torque converter locks, up and 80 is a tidal wave of torque. When my Dad originally bought it the car had been ordered by the dealership as a demonstrator with dynaudio premium sound, summer, winter and protection packs and the polestar, but once my old feller got wind he'd bought it before it had even been delivered.
I've always fancied a T5 or R of some flavour, 850, V70, C70, but I guess it'll never happen now.
But the best bit about Volvo 5 pots of any kind is that off beat warble. Not quite a V8 sound but unmistakably more exotic than a four.
Edit - I've never looked into it seriously, but I do believe the quad exhausts on the R Design means the standard accessory towbar doesn't fit.