I can't find anywere in the internet that say that the RR doesn't have nikasil. Neither is confirmed. I have found someone that sold oversized pistons for the RR W/W but not the SP. This is what it says:

Honda NSR 150 RR W/W - SP piston repair kit. For steel cylinder use all piston size.For aluminium-nicasil cylinder SP type use only STD size. piston Option STD 59 mm.

piston Option STD 59 mm. or +0.25, +0.50, +0.75, +1.00, +1.50, +2.00

The part catalog of the RR doesn't say anything about different piston sizes. I don't know if Honda has that in other catalogs from other models...

So... You say that the RR has steel barrel. If I ever dissasemble the engine I'll place a magnet and check it out. Hmm... I could have done it when I serviced the RC system. Too late.