[Ray beat me to it with a more comprehensive answer, but we said similar things
]
Hi Paul,
Your system is approx one third the speed of Mike's based on the small difference in image scale, a little more obstruction and the larger difference in QE (for Ha the QE of the KAI-11002m is down around 30%.)
I'd agree with Colin that with 20 min NB subs you're going to be taking a big hit from read noise, which is up around 12e- for that sensor.
Longer subs would help, but you're at a big disadvantage based on QE and read noise. It is definitely the sensor that is the weak link in the chain.
Cheers,
Rick.