If you have issues with injuries, I'd see if the shops you looking at bikes offer fitting services, then talk to the fitter and explain.

My gut says you may end up staying on your Hybrd. Efficiency on a bike you can't ride because it kills your back isnt so efficient, is it?

I'm guessing your Sirrius has 700x32mm tires, so if you went to something a little narrower, you could see an increase in speed not sure now the ride would be affected... You'd almost assuredly "feel" more of the road.

I know the Dolce / Ruby are one of the more relaxed geometry road bikes, along with the Canondale Synapes, so I'm wondering if you've tried out any road bikes? How did it feel? Are you able to take a road bike on a longer test ride to see how your back reacts? You mentioned performance boost and wheels.. How much weight would your $400 trim from your current bike? I know wheel upgrades on my hybrid would only give me back one or two pounds at most in weight. For $400 (close to $100 of that in new tires) wouldn't get you the featherweight wheels like the pros ride, so I wonder just how much bang you'd get out of the rims. IMHO.

Good luck!

Shannon