What size cassette are you using Kaz and is the Rear mech long cage or short as a long cage should give pretty much the reach you need across the cassette provided the gearing is somthing on the lines of 11-28 ..
Because RSX was only designed for the 3mm shorter cassette of 7 speed, if you have a very thick dropout (e.g. early Alu or Ti) it can lack the reach to go all the way inboard.
Generally Shimano put in a load of lattitude and it's fine.
it should work fine - cable pull is the same for 7 and 8 speed shimano mechs, and in 7/8 speed everything was still the same for both road and MTB. Even if the dropout is wider than a standard road dropout there should be enough adjustment in the high/low adjustment screws to give you the range for an 8 speed cassette (if you're right on the limit though it might not reach the last sprocket). And a 24t sprocket is smaller than the largest "allowable" for that mech.