Agree to disagree on the clothing-as-protection, it sounds like it works for you where it hasn't worked for me.

~12pmh avg is on the slower side unless your commute is a lot of stop-and-go or uphill. At least, it's slow in the reference frame of lycra-clad cyclists. Aero matters more the faster you go. If you're really concerned with speed... well, you know where that's going. Something about an engine.

(EDIT: I see Phil made this point above already... I'm a slow typer)

Since you're showering at work anyway and want to have less drag, go for it. Sorry I can't suggest any specific pants but it's easy to find fully sleeved upper layers- doesn't need to be bike specific, any of that moisture wicking base layer type shirt will work.