If I wanted a longer tube, I'd talk to a gunsmith that does some Ruger DA work, and talk about having a custom barrel done up to whatever length you want, 4", 3", whatever, of better quality than stock and whatever sight type you want.
Clements modifies GP100s on a regular basis, so he could prolly wrench on an SP101. Ditto Jim Stroh at Alpha Precision, I think? They'd be the first two I'd check with. I doubt it would cost much more than swapping in a factory long tube.
