Spark runs L sizes. Keep in mind that current Spark's do not replace the Voile system. They just work with it, lightening your load a ton and making for much superior binding performance. Of course foot size could play a role if the bindings don't fit. Overall though a split specific binder is way better than using the plate your own binding combo. It's an amazing difference. As far as upgrade goes, it seems to run from smallest to biggest upgrade in downhill performance, Voile Lightrail, Spark, Karakorum. I am omitting the Spark Newton, as it looks like it will offer the same if not better performance than Karakorum, but it's not out yet. I haven't used it, so it remains to be seen. I will say that from what I saw, it looked like a pretty solid design. I am excited to try it.
So you may be right, that adapter plates and his own binding are the way to go. I would make dang sure that none of the split specific bindings are going to work first. It is that big of a change.