If you are really loving the Grado sound...then staying within the line would be the best bet to get improvements. That said,
www.goodcans.com has really nice writeups about the various models, particularly the SR60's and the RS-1's and GS-1000's. I think what they have written is pretty spot on. Starting with the SR60's (the SR80's in my opinion are slightly better and start to show some signs of improvement when used with an amp) one gets a heck of a lot of sound for the money. I'd wager that I'm enjoying 75% or so of what the RS-1's offer, maybe as high as 85% depending on the type of music I'm listening to.
At 1/10th the cost...I'm at a pretty sweet spot, 3/4 the performance or more at 1/10th the money. Spending to go higher up in the chain will offer performance improvements but 50% more? Not overall, but certainly in some areas of reproduction and heck maybe even more than 50%.
The SR60's already offer excellent resolution and a wonderful midrange. They can get a bit harsh up top and their bass is plump (but darned fun). The SR80's have a tighter bass, and more controlled top end while retaining the lovely midrange.
Jumping up to say th 225's, you get tight solid whomping bass, that same wonderful midrange with more instrument separation, more resolution and a richer sound all the while having in my opinion, the best top end of the Prestige line.
What doesn't dramatically improve then is the midrange overall, the imaging and the soundstage. Of course, dropping in an amp one will notice the 225's seem to take better advantage of the cleaner power than the SR80's.
So that leaves the Reference and Statement Series to really demonstrate what is possible from Grado.
Larger cups = larger headstage, and quite a bit larger soundstage. Resolution is incredible at this point and the tightness of the bass, wonderful top end (no harshness) rich tone and timbre and the layering of the sound is so much better in my opinion. So, are these traits that you are looking for? If not, then spending more may not equate to ultimate satisfaction. You might not view these aspects of the sound as ultimately important (though most around these parts do of course). It seems that to many of us, maxing out these aspects of sound is really what audiophilia is all about because they are notoriously difficult to achieve. However, this may then equate to a full 50% or more in enjoyment of the music because it sounds that much more real. For me, though many transducers can get me 75% or more of the sound...it takes the really good stuff to approach the 90%+ and therein lies the cost. Getting closer to 100% (if that is ever to be possible) will at least at first...cost an arm and a leg. For many that is what they are trying to achieve, hence the megabuck systems.
That said, don't discount the greatness of the SR80's. Definitely a great set of cans, but if you want even more of the same and some extra stuff you aren't hearing...go for the 225's or if you can splurge...the RS-1's. Of course for the ultimate experience...grab the GS-1000's, but they sound a bit different from the "Grado House Sound"