Made my 500 go much better by addressing her stiff throttle issue. First I changed both throttle cables but that made no difference at all. So I sprayed WD40 on all the carbs linkages and pivot points - at first it seemed it didn't help, but a little later I realised it must have slowly seeped in, and had in fact totally fixed my issue! And what an enormous difference it made, she literally feels faster!
Once warmed up she still paggs out at small throttle/low revs, like when taking off, even with the mixture screws 2 turns out, so I guess I'll try swapping the #40 pilots for #38's next.
John