Warning, the following is based entirely on dry theory, rather than practical experience
I just ran the numbers, and with that sensor you would be imaging at 0.46" per pixel without any reducer or extender in place. The diffraction limit of your scope is 0.58", so even under the best conditions you likely would not gain much by exceeding f/20, with an effective focal length of 4000mm.
In perfect seeing conditions, a 2x barlow would result in imaging at f/25, 0.23" per pixel, and just over the Nyquist sampling resolution of 0.29". In more typical seeing, you would likely be over-sampling already.
I'd personally suggest a 2x Powermate if you have the budget, but bear in mind that it will likely just result in muddy images under less than perfect conditions.
As a practical example of what you can expect with that camera and scope, imaging Jupiter tonight would result in a disk diameter of 89 pixels.