The XT11 is a standard flat bed side-by-side 2 place trailer. No fenders, etc. So spacers won't do anything. Diameter is the problem, not backspacing or width.
Standard 18.5x8 tires that come on that trailer are about 18.3" diameter. The next natural upsize to these standard tires are 20.5x8-10's, which are 20.4" diameter. I can confirm firsthand these will fit fine as that's what I ran under my Triton 2 place..I ran a pair of them for about 25,000 miles, they still looked new when I sold the trailer. Up from that you could consider a 5.30x12" radial. Great tire, but 21.9" diameter. I'm not sure it would clear the underside without rubbing in bumps under load. It might. Never tried it.
Anywho, I agree the standard 8" load C tires should not come with that trailer from the factory...they're undersized and will wear uneven if you load it with 2 sleds usually. For one sled they're fine. So I'd suggest going with 20.5x8-10D tires for that trailer. At load range D, they will have more carrying capacity than the trailer could ever handle. Run them at max sidewall pressure, which is 70 psi. And if you really feel you want aluminum 10 inch wheels, they do exist. Random google search:
http://recstuff.com/10-inch-aluminum.aspx
That said, I'd just go with bare galvanized steel wheels. IMO they're too small and hidden/buried under there to really care about looking good...but many seems to care more about trailer bling than I do. So if you're into looks, the alums can certainly be found.
Whatever you do, make sure you put the tires on before you select a hitch rise. Larger tires raise the bed a little bit, and you might need more rise or less drop on your hitch shank to level the trailer.