Little light fwd cars like ours will always have lift off oversteer regardless off the RARB being fitted or not.
My car seemed very neutral on track once the tyres were upto temperature - before that there was a bit of oversteer as the rears naturally take longer to warm up. I do have a thicker front arb too (and coilovers, braces, poly bushes etc).
A bit of oversteer is not a bad thing either as it gives you more adjustability than persistant understeer.