I have been using the Advanced Heavy Cruise - Retrofit and I like using it. However if I am to improve my damage and defences will I have to upgrade to the Vice Admiral tier ships or will my favourite ship perform just as well?
I don't see a reason to upgrade. The Excelsior is awesome. It's probably the best turning cruiser in the game which means you can actually use your torpedo launchers, it's got a decent tactical setup, and it's just awesome all around.
On my VA engineer, I dinked around with the Odyssey for a bit and realized I was trolling myself. Back to the Excelsior I went.
The Excelsior-R and VA ships are all T5, in fact IMO the Excelsior-R is the better cruiser. Better turnrate, better dps machine when compared to the other cruisers. Keep in mind ships you get access to at Rear Admiral Lower Half and at Vice Admiral are all T5 ships.