Possibly a daft question – can I fit a standard 9 speed road cassette on my hardtail commuter and leave everything gear-related unchanged (chain (KMC)rear mech/shifters (SLX) and rear wheel hub)?

Cheers for any advice.

bbA

Yes. !

yes. Your hardest gear will still be the same (if you have 11t smallest sprocket) but the narrower gear range will probably just mean more like the 'right' gear for your legs on long rides. I have an 11-28 on mine, ride up a bastid hill to work and I don't miss the 30 or 32 one bit.

my oldschool xc mates all have road blocks on, why not go crazy and convert to a 10spd?

The chain will be a bit long but go mad!

Yes, loads and loads of downhill guys use them. If you're commuting you have no excuse not to use one.

Yes, but for best performance, make sure you shorten the chain correctly and wind in the B-tension screw on the mech properly for the reduced biggest sprocket on the casette