That's an extremely important point.
I tell people: If you want to help startups, the tax rate does not matter hardly at all. While you are still a 'starup' you will pay very little in taxes, just 'cause you will make very little money that is not re-invested. The tax complexity makes a huge difference, though. Now, I'm sane so I pay an expert to deal with that sort of thing, but a complex tax code is worse than the costs of paying another expert.
A complex tax code, first, massively increases my risk. I know two people who will likely be in debit for the rest of their lives because they tried to do their taxes themselves while running a business. Now, I've got a professional, but there is always a chance they will screw it up. I'm on the hook for the money, no matter what. I can get out of my building lease and loans through bankruptcy, but tax debit is forever.
Next, a complex tax code makes it harder for me to plan out how to allocate my resources. With a complex tax code, it's often more profitable to play accounting games than to actually build something people want more efficiently than the competition, and I think that impoverishes us all.