T5s and temperature

Hi all

I have a 4ft, 18" deep tank that is a (UK) temperate tank. I think of
it as a reef tank as I have a DSB and live rock, and much invertebrate
life on the rock (although the inverts aren't as spectacular as the
coral pics I see links to here).

Obviously I don't have to worry about the high light levels you need
for corals/anemones, but my current lighting is not enough to grow
much macroalgae, which I would like to do.

I have a 4ft standard flourescent (which is a 60/40 actinic/white
light mix IIRC).

I've been looking at a twin set of 3ft T5s (one white and one actinic
maybe)?

Will this give me a lot more light than the 4ft flourescant?

And importantly, will it give off a lot more heat? I have an enclosed
hood, and I'm right on the limit of acceptable temp for some temperate
species (especially in summer) and I can't afford a chiller.

Thanks in advance for any help.