I do believe you have found out the same thing I had to... a 166 mhz gp32 doesn't always equal 166 in snes9xgp or any program for that matter...
I can run 166 in everything but snes9xgp and DrMD.
Get the clockspeed tester program... if it = 166 mhz for 20 minutes then you do have a 166 unit... and you'll just have to get used to 160 mhz in snes9x, it's only a 6 mhz difference... not even noticable.