Hi Oliver,
It's nearly impossible for any reel to be much of an improvement over an 08 10k Stella! At this level and price bracket, it's all fairly minute differences between the shimano and daiwa top tier reels these days so unless the Stella is in need of replacement, I'd keep running it.
The only noticeable benefit of the 5500 exp is that you don't need a larger spool for pe8 work. Alan Hawk rates the expedition as the best reel he's ever used however read in to that as you like. I find his heavily engineering based reviews much less important than reviews from people on here that own and punish reels. That's not to take anything away from Alan as the man knows his spin reels.
I'm more of a Daiwa man but if I were in your shoes, I'd be considering whether the Stella needs replacing?
Cheers
Rich