SRAM and Shimano cassettes use the same spacing for 7- and 8-speed (I'm using a SRAM 7-speed cassette with a Shimano derailleur and shifter on one bike and an 8-speed Shimano cassette with a SRAM derailleur and shifter on another). I'm 90 percent sure they're the same for 9-speed too - a guy I ride with runs a 9-speed SRAM cassette and a Shimano derailleur, and it's perfectly adjusted.