I just got a pair of the 100-flex Atomic Overloads today. I have wide feet and a long big toe, and the Overloads and Atomic LF 90 were the only boots that felt tight and comfortable at the same time. Fit-wise, these two felt quite similar. If you can't find an Overload to try on, try the LF 90 and it will probably be close.
While boot shopping, I found several boots that crushed my instep, particularly the Salomon Ghost 90. Based on the boots I tried, I would say that the Overloads do not have a small / tight instep area. I didn't try the SPK because the boot fitter said it was a poor choice unless you are a hardcore 100% park kind of guy (and I am not).
I tried the Full Tilt Booter and Salomon Impact 100. Both felt just a bit narrower than the Overload and didn't have the same space in the toe box. No instep problems in either. The full tilts had a super-nice cushy footbed and I liked the way they flexed, but they didn't keep my heel from lifting as well as the others. The Overloads hold my heel down really well.
As was already mentioned, the Overloads may fit big. I was a 28.5 in Full Tilts and Salomon, and a 27.0 in the Overloads. My old boots were a 30.0 (I was 14 or 15 and I think my parents wanted insurance in case my feet grew) so the only bad thing I can say about the new boots is that I now have to get my bindings remounted.