Wheel weight vs. foot print. The Focus is a heavier car than say a Civic and needs a wider tire. The problem with running 15" and 16" wheels is that there aren't a lot of tire size choices in the newer competitive rubber that will give you any advantage over that Civic that's running the same size tire. The alternative is to move up to a 17" wheel where you can run a bigger tire than can be stuffed into the wheel well of a Civic.
You might consider trying the following...
17x7.5 Rota Slipstreams with a 45mm offset. These can be found at zxtuner.com for about $140 per wheel.
Dunlop Direzza Sport Z1 Star Spec in a 215/40ZR17. These run $128 a piece at Tirerack. Just as much grip and heat resistance as the RE-01R's, but they have a stiffer sidewall and seem to do pretty well at full tread depth. The Dunlop's also tend to run wide on their sizing.
Throw on that Saleen suspension that Massive is selling for $500, some LCR street camber plates and kick some STS butt and still have a very livable daily driver.
Just my .02
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