Using my f/10 SCT at f/4

As I do astronomy on a budget, I like to see what I can fudge together or get away with. I do not have sophisticated tracking equipment on my SCT, just the R.A. motor and a manual guiding 1 x sidereal rate hand controller. This would suffice if I were to enjoy spending hours typically at -2ºC, hunched over an eyepiece, trying to counter the effects of wind and drift in my motor versus the sidereal rotation of the earth. I can't be doing with this and my knowledge of optics tells me that at f/4 compared with f/10 optics, the light gathered per area is 2.5x2.5=6.25 times greater thus shortening exposure times. I got hold of a second f/6.3 field flattener and after many long attempts with various coupling adapters I found a combination that worked with the focus at the end of its travel. I had to use a 1 1/4" adapter (I will get hold of a 2" version ASAP) so the vignetting (edge-of-field clipping) is quite severe; still it gives it a lovely 'through the telescope' quality! This is the Pleiades with an effective focal length of 800mm and aperture of 200mm in a single shot of about a minute.