Pooks-
It's my opinion (because this is all theory, each voice a new opinion, etc) that a heavy bike will produce more exertion.
If you ride a heavy hybrid an hour, vs. a light road bike an hour, given the same amount of exertion, the road bike will take you much further, faster.

The question lies more in "do you want to do smaller neighborhood rides, or go for an hour spin and manage a 15 mile ride?" Are you comfortable on your hybrid?
Anecdote:
My transition came because my heavy (32#) mountain bike's frame was too small, and not made for what I found my body wanting to do. With saddle all-the-way-back and bar ends, I was getting low-and-tight to use my whole body to try and get appreciable zoom- but as the bike was too small, and very heavy, I ended up hurting all over from what should have been a fun little ride.
That said, it's still a great workout taking her out on the singletrack for 8 miles at, oh, 4-6 or 10-15mph (on hills)
It's taken me three years, from the time I started riding with Mz Cakes in the spring of '04 to now, to be at a place where I can get my zoom on with my mtb but feel that, mechanically, physically, geometrically, she limits me in the directions I want to go... but her weight and those limitations have made me a stronger rider, I think, than I would have easily become otherwise, and I will probably continue to use her to commute during the winter because she's heavy, strong, can put up with the elements and will provide a decent workout for the short distance I'm going.