I think the A5's sound a lot better, even in a small room. I had the A2's for about 3 weeks before upgrading to the A5's. I am happy with the switch, primarily due to superior low-frequency reproduction.
The A5's produce a much bigger and more defined bottom end, which is to be expected when you go from a 2.75" woofer to a 5" woofer. The A2's would sound congested when thrown lots of sound in the 100-500hz region, which is a pretty wide (2+ octave) range. The 50-100hz range lacked punch and volume. However, I should note that I felt like the A2's handled musical material in the higher frequencies very well. Vocals, trumpets, and flamenco guitar are some examples in music that the A2's handle extremely well (given their size and price, naturally).
I EQ'ed out the infamous A2 low-midrange hump which helped a lot with the congested sound in that region, but it still lacked overall impact down low. After awhile my ears adjusted and it sounded good, but I'd already decided to upgrade since I wanted the best full-range sound possible (all the way down to almost 20 hz when paired with a sub). Accurate full-range reproduction makes ALL music better, but it was especially important for me since I listen to a lot of "electronic" music, for lack of a better term. The A2's alone don't do that genre justice.
I also like the iPod charger and audio-in ports on top of the A5's, which is a nice feature you DON'T get on the A2's. It's very convenient to play music off my iPod without having to disconnect my computer first. I don't always want to deal with the noise, power consumption, and start-up times of my full desktop, and it's nice that I don't have to dig around in the wire nest behind my computer every time I want to hear something off my iPod. The A5's with my Martin Logan sub can produce a very large and amazing sound... sometimes its hard to believe its all coming out of the tiny little iPod on top of the left speaker.