What is your typical star size with the 70mm f6 currently? 3 arcsec is only 6.1um with your existing setup so you are potentially undersampled already so there would be no advantage going to a lower focal length to beat the seeing. (I think potential gain is only marginal anyway as all you do by moving to a shorter focal length is reduce the camera noise contribution, the star and sky background counts stay the same).
The extra aperture will gain you some photons though (Area of C6 is 608 cm2 allowing for the central obstruction compared with 154 cm2 so ~4x or ~1.5 magnitudes)
Robin