Of course you will see better performance. That comes with the fact that you have a higher bandwidth, period. BUT the VPN overhead does take its toll on the speed of the terminating devices, generally firewalls or routers. Depending on the quality of the equiptment you may notice a slight slowdown in throughput. But that will be more than offset by the increase in speed.
Now you do have other issues, like the fact that the cable system is shared, so those speeds mentioned before are not *garaunteed* like DSL. (note that DSL is still shared, but at the CO, where the big pipes are) So depending on the local circumstances, you may not see any improvment, or even a decrease in performance. This being said, you *should* have faster transfers, and depending on what network the remote end is on, you may have better response times as well. Then again you may get slower response times and higher average speeds, thereby not really helping much at all.
I would get cable in, test it, then see which is better and stick with that. The overhead for a VPN tunnel was like 4-6% on T1 speeds if I remember correctly. (note: I often do not remember correctly) If these percents are correct then doubling your speed and losing 6% is still better. I hope something in here helped in some way.
EV