That the two of these lens are about equally sharp in the center is not surprising. The "L" should have a distinct advantage in the corners on a full-frame camera. IMHO, on your 1.6x 30D, you're paying a lot more and carrying more weight than you'll gain in sharpness. What you do get is better build, better focusing system, slightly better quality bokeh, perhaps more Canon L-like color rendering and slightly better contrast.
The way I see it, the 24-70 should be due for a "refresh" by Canon soon. Meanwhile, the 28-75 is probably one of the best Tamron lenses ever, for not a lot of money. It's not unlike the 85 1.8 versus the 1.2L. I can understand your disappointment.