Night and day! I went from the 555s to the *Spec, and for our purposes, there's no going back. Think of it this way: 300 to 200 treadwear is pretty much the same change as going from 400 to 300.... like from the stock P-Zero Nero to the 555. You can expect essentially the same increase in grip. If you dip much below 200, you're going to be looking at "race rubber," and they will wear a lot more quickly than you're used to. Think Nitto NT-01, Toyo RA-1 or R-888, and that will give you an idea of what the entry-level race rubber will be like. Make no mistake, a 200 treadwear tire will NOT wear like a standard 40,000 mile tire, but it should give you a lot more wear than the race rubber will.
GRM just did a tire comparo between the Kumho XS, Dunlop Star Spec, Falken RT-615K, and Hankook R-S3, and between them, the road-course laps had less than a 1.6 second spread, which really isn't bad.
For what we do, though, it's not a matter of treadwear, it's a matter of heat-cycles. When I swapped my Nittos for the *Specs, I still had boatloads of tread left, but the tires were cycled out. They just didn't grip any more, and got greasy after maybe 5 minutes of pushing them on track. When new, the Dunlops would last through a 30 minute session with no problems whatsoever, and I'll know that it's time to swap them out when they start getting greasy after 20 minutes. With the Hoosiers, you can tell that they're done when they just don't grip anymore, and as I mentioned, that happens for me after about twelve 15-20 minute sessions. Not days, sessions. I'm doing something like 25 track days this season, and I'm anticipating wearing out three sets of Hoosiers, but I'm betting on only one set of *Specs for the whole year. Because they're only a second or two slower than the Hoosiers, I run the *Specs to and from the track, run them for the warmup session, and sometimes for the first timed session (NASA TT), just to see where I am in the field. If I need the extra speed, I'll slap on the Hoosiers, but every time I do, it's like lighting a $100 bill on fire... Otherwise, I run the *Spec, and just have fun.