Hi Andy, I have a Sigma 150. It's got a super light flywheel and race clutch as fitted by Stuart, which has sharpened it up nicely, but the 175 felt far more powerful (as you'd expect). I can see why reviews are positive. I don't think Stuart has a customer who hasn't been happy with what they've bought, and I have always been impressed on my passenger rides and now with an actual test drive. If I had a few more quid and an emptier priority list I'd seriously consider it.