I made and posted that chart at HSU's forum years ago!

I also happen to have the 4ohm driver sitting here on a shelf and the HSU VTF-2.1 subwoofer. I purchased a replacement driver directly from Hsu.
I think the HSU driver might have a higher efficiency. If the TC Sounds driver is of lower efficiency, you will also lose output capability. With the change from 4 ohm to 8 ohm you will lose about 3 dB of maximum output.
The amp in the HSU is designed specifically for the HSU driver to created a flat frequency response in that enclosure. The TC Sounds driver might require a ported enclosure with different volume and port length. It might also have different rolloff characteristics and high pass filter requirements. However, given the low powered amp, you probably won't have enough power to cause any actual problems and if you can EQ to the listening position you will be fine.