I think as long as it is period, Phil, then a Ninco 911 is certainly no better than a Team Slot or Avant Slot Alpine or an OSC/SRC Capri - those are the default choices now whether running classics as a racing class or on the stages. There's no easy way of defining it, though, because you can say 'no whale tail and wide arch 911s' only to be blown out of the water by history!
The beauty of the SRGB regs was that they weren't actually complex at all. Get a standard inline rally car and compete with it. They didn't need the same level of knowledge or fettlement to be competitive as you do with, say, a GT class on wood.
Personally, I don't find any massive difference in performance between an inline, sidewinder or angle winder with the same basic motor but 'inline only' did prevent arguments. Of course with pro cars that have magnetic motors it's a different thing altogether but for Ninco, Scalex, SCX etc. using regular motors there's precious little difference between layouts.
As of now, I'd say the cars to beat in each class are:
Classic: Pro: Avant Slot Alpine, Stock: Ninco Porsche 911
Eighties: Pro: OSC Peugeot, Stock: Ninco Lancia 037
Modern 2WD: Pro: NSR Clio, Stock: Ninco Clio
Modern 4WD: Pro: MSC Impreza, Stock: Ninco Lancer
Or in other words the best Pro cars have the best magnet motors and the best Stock cars have suspension and sprung guides!
Looks like the new Scaleauto hatchbacks (Peugeot 208, Hyundai i20) could shake modern 4WD up a bit. And OSC is breathing on the SRC Porsche 914, which could be worth a punt. Still plenty of life in slot rallying anyway.