It doesn't matter what your math says. You are not going to make 8% a month forever.
You are short gamma. As a matter of fact, if you're managing to get 8% a month doing this, you're very short gamma. Your vega would also be very deep in negative territory.
I've only been doing the same thing for a few months, and I've already figured out that 3% is relatively OK, 4% is pushing it, and 5% means you have to be ready to constantly adjust somehow. I'm between 3 and 4 in these first few months, and already I have to figure out how to adjust in my third month, and I'm not sure I'll be successful either. So I know even that 3 to 4 figure is purely hypothetical, pending my success in coming up with a workable, consistently effective strategy for squirming out of months when the market goes against me.
I can't begin to imagine what you would do or how short gamma you have to be to claim 8% per month. The market isn't just giving you that. I hope you have a very good strategy for when things go against you.
And I will say this again, because it's worth emphasizing: every bp you make over Treasuries you make because you are taking on risk. At 8% per month, you're taking on a monstrous amount of risk.