I also have a C11 and image with an ASI462MC. I'm no means an expert, but I've collected some decent data at ~300-310 gain. Others have gone higher than that with gain with even better results, so play with settings and see where that places you in terms of data collection.
Currently, for Mars with an ROI of about roughly 300x300 and gain at roughly 310 I'm getting ~250 fps and a histogram of ~70%. As others mentioned, you could go lower on the histogram to around 60% (or increase gain) and squeeze in some more fps. 300 fps is definitely doable, but you should be getting more than 180 fps with your setup. As others have mentioned, there is a spectrum of what is "correct." I capture for between 4 or 5 minutes depending on what I'm seeing on the screen.
The 2.8x barlow is quite a lot for that camera. I'm not saying it couldn't work, but you are oversampling with that chip size. If you have a 2x barlow (or less) laying around, try that. You should also give it a try without a barlow. You wouldn't be undersampling by very much given the chip size at your f/10 native focal length.
If you prefer SharpCap (nothing wrong with that!) you can create your own custom ROI as you can in FireCapture, though you cannot draw the ROI on the screen as you can in FireCapture (click on "Capture Area" button -- scroll down to "Custom"). The neat thing about SharpCap is that you can then move the ROI around the entire possible capture area if you'd like by clicking it and dragging it. It isn't a necessary feature by any means, but a nice perk. I switch off between SharpCap and Firecapture regularly and they each of their benefits. The program you feel most comfortable using is the one that will produce the best data. Good luck and don't fret; there is still plenty of time to get great Mars data.