Had the same problem today and fix is not that hard :)
In interface builder add a UIView with the exact same size as the UIProgressView. Add your UIProgressView in this new UIView.
Do the following for your UIView if you want corners:
1) Set cornerradius to the half of the height (e.g. 30px height progressbar means 15px corner radius UIVIew))
myView.layer.cornerRadius = 15;
2) Set mask to bounds
myView.layer.masksToBounds = TRUE;
3) Set clip to bounds so nothing can come out of the UIView
myView.clipsToBounds = TRUE;
And now you have a rounded UIProgressBar with the height you want.
If you don't want rounded corners just add a UIView as the same way but only change the background color to the same grey as the UIProgressbar tracktint.
Goodluck!
-Sjoerd