I agree with Pundit there is massive room for efficiency. Ban cash - in hotels, matatu, soko, etc. It's easy to ban cash over time. Any permit business like a bar or restaurant or chemist make it illegal to accept cash. Only mkokoteni guy and mama mboga or small kiosk needs time to go cashless.
Revenues would double. After that you can decide about revising brackets to flat rate or whatever. Not before.
I think KRA is progressively working on this and nailing evaders. I saw they hired a Chief Manager Big Data recently. They need AI & Analytics and modern tech to sniff out all sources and seal loopholes. With time Kenya should wind up cash completely by slowing the note printing. Even K-street should pay tax. Mop cash off the streets and people will pay.