OTB, you know that even sora and tiagra shift fine right? I raced with 105 for a while and it was fine. You just have to learn to tune it up and then it works. The rear shifting isn't a big issue, but the front is finnicky at times with the cheaper setups. If you're not using a triple then it's rarely an issue though.

I'm not sure what your racing background is, but Jrennie's ridden his share of bikes and done his share or races.

An inexpensive frame is inexpensive for a reason. Part of it is because it's someone else's outdated design, and part of it is because the manufacturing costs are cheaper. They skimp somewhere, materials, craftsmanship, etc.

I'd say handling/geometry/fit are more important than having expensive parts on your bike.