I rode a 9-speed Tiagra/105 OCR2 for a few years, then upgraded to 105 on a CAAD9 and now I run full Ultegra. I've noticed almost nothing between the groups (do not race). Actually, 10 speed is more finicky, requiring adjustment much more often than 9 or 7 speed bikes I've ridden. My suggestion to the OP is upgrade the drivetrain when you crash the bike and destroy those parts. Another thing to keep in mind is that Shimano uses a trickle-down strategy with all but its lowest groups - so 9 speed Tiagra should be on par with Ultegra-level 9 speed from back in the day.
Actually, I partly lied: when upgrading from 9 speed tiagra to 105/Ultegra I got an external BB Shimano crank which was over 9000 times better than the square taper Truvativ cranks that came on the OCR. Stiffer, smoother, better.