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My WO Megrez 90 would give you 2.03 arc seconds per pixel with that camera and a 0.85 reducer, or 2.16" with a 0.8. That would seem for the most part to match your seeing, however remember that according to Nyquist's theorum the optimal sampling for resolution is to sample at half your resolvable detail level. Therefore, if you are after resolving detail in small DSOs and not extended objects, you'd be better off at the resolution you get from your VC200L with reducer. It would give 0.93"/pixel on the 450D, with a roughly 1.1x0.75 degree FOV.
As you already get 1150mm with the VC200L/reducer combo, you might consider getting a 600-800mm scope to supplement that for larger field views. In that range we have scopes like the Megrez 90 or any of a number of other refractors, or significantly faster Newtonian designs like the Vixen R200SS supplemented with a coma corrector (1.34"/pixel).
Both of those options should be well within the capacity limits of the Sphinx.
Regards,
Eric
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