I have used them in the inner positions...they end up grenading much like the stock wheels have. As Mooseman stated, they are wider, but also beefier. Whether that is a positive or a negative in this application...I just don't know.
This year, I installed thinner wheels on the inners, I think they are a knock off of the Skidoo idlers, but made by Kimpex. I got them through Royal Distributing.
If these don't work out, then I think I'm gonna try the old metal idler wheels...and if they don't work out, I'm buying a Skidoo.
Regarding getting the Excell ones, I bet they'd be a difficult find, but you never know, maybe they have a huge stockpile of them in the works, but seeing that they aren't just releasing them yet, I wouldn't bet on it though. See if you can order them now.