Thinking about shoes

I normally wear Keen commuter sandals and I love them. I have a funky tan, but here in Texas it is a blessing to have the air through my shoes. I do notice that rides over about 20mi I really notice the flex in the shoes. So far it hasn't been a huge issue since 35mi is my max, but as I go farther I know I'll need hard soled cycling shoes. I'd planned to get mtb shoes so I could walk easier in them.

But now that I've signed up for a triathlon in Oct, I'm wondering if a pair of shoes that didn't flex at all would contribute any to a faster ride? Should I invest in the shoes now? Do you think it will make any difference?

btw - this is a sprint tri so I'm only riding 10mi.