The allure of the Mak-Cass is real to me, as my observing interests are mainly planetary. They are short, easy to mount, and their long F/L makes it easy to achieve the magnifications needed. I owned a TEC6 f12 which I compared extensively with my 4" and 5" apos. I concluded that for planetary it was likely equal to a 4.5" apo. My experience with the TEC6 led me to purchase a TEC8 f15.5. I used that to compare with my 6" and 7" apo and concluded it was probably the equal to a 6.5" apo for planetary. Fast forward to today and I have a TEC7 f15. Given the established data points, if one extrapolates the performance it would therefore be a safe assumption that this quality 7" MC would be the equal of a 5.5" apo for the planets. Certainly the TEC7 would outperform a 5" apo. I've had mine out with my 130GT and can confirm this to be so. Again, I can attest only to planetary.
Some notes from the field:
1) MC's need significantly more time to cool down. As such, not an optimal quick peek scope.
2) MC's are more prone to be affected by seeing by virtue of the CO. On bad-seeing nights, one would probably do better with a 4" apo than a 6" MC. Same holds true for a 5" apo vs a 7" MC. Apos are more consistent more often.
3) The TEC8 f15.5 gathered significantly more light than the 6" apo. Whereas I found Jupiter too dim at 300x with the apo, it was still pleasing with the MC.
Just my experience, hope it helps.
Interesting comments. My experience with some large Maks (I own 6,7 and 8 inch Intes Maks) is on planetary low contrast the 7 inch Mak (IM703 f-10) will not quite keep up with my Zeiss APQ 130/1000 which is a true apochromatic refractor. The 8 inch Mak (IM815 f-15) will beat out the Zeiss APQ130/1000 but not by much. 815 has 24% CO and the 703 (32%).
I wonder when folks say a Questar 7 with 33% CO beats out a 5 inch Apo on planetary whether the apo being compared are as good as say a Zeiss APQ or a Tak TOA on the low contrast planetary stuff etc I have yet to see my IM 703 do it. But then again seeing is believing. Maybe I have yet to get the very best of seeing where the 7 inch Mak can win??
All three of my Maks are collimated and I live in a temperate climate.
On deep sky the 7 and 8 inch Maks will go deeper than the APQ 130 refractor, but the refractor still shows some of the hard to see details in nebulosity due to it's high contrast for it's lower image brightness.
I spent quite a bit of time with my 6 inch f-10 Mak IM 603 over Xmas when travelling around in a motorhome. I was really pleased with the images it produced. I did not have a refractor next to it but suspect it was producing deep sky images not far from what a 4.5-5.0 inch apo would produce. I did not use it on planetary. At a star party other observers using large Dobsonians (10-20 inches) kept coming back for views. Although dimmer the IM603 was delivering razor sharp images.