i currently have a 105/ultegra mix. it shifts smoothly and i like it a lot. i'm especially glad to have shifting in the drops.

HOWEVER, my old bike had tiagra rear and sora front derailleurs with sora shifters. i actually think the sora shifted more reliably under load than the 105 fd on my current bike, possibly because they designed it to be more "idiot-proof" since they put it on lower-end bikes. so yeah, 105 is certainly nicer, but as long as it's all properly adjusted you shouldn't really have any problems, even with the sora stuff.