My two cents:
I was in the same situation as you. Had an 8, was disappointed with lack of detail.
Bought a 12.
Sitting out here now, on the wireless, looking at Mars. I see nothing but an orange blob. Occasionally, I spy perhaps polar cap, but could just be my imagination. There is a lot of glare at the moment from mars.
I use the plastic cap over the scope. I see a less bright, uniform blob.
Regarding some of the suggestions here.
Scope has been out for around 1hr 30. No fan running. Collimation I judge to be slightly off centre.
From an 8 to a 12, there isn't that much more focal length so it isn't that much bigger and with a bright planet, light gathering isn't exactly the most important thing either.
So I guess I'm saying.. maybe think twice about going up. Try to polish what you've got because I did what you want to do and it didn't help
(It did help dark sky observing).
I should also point out that I am using cheap EP's. EP's that i find difficult to focus, really, always a bit of a bright ghost image overlapping. Hard to get a crisp image. I can see detail but really, that my biggest problem, the coma/ghosting.