It's definitely useable
Be warned, large PVC ports have a seductive nature that can sneak up on you....
You will find that a large dia pipe needs a large box. Here are some figures (sorry about the metric..)
for a port 187.5mm dia and 600mm long:
a tuning frequency of 35hz requires a box size of 111 litres
a tuning frequency of 27hz requires a box size of 154 litres
a tuning frequency of 20hz requires a box size of 300 litres
pipe resonance 286hz
I have built a dual 10inch sub with a pair of 6inch ports in a 130 litre box tuned to 28hz. With 240w applied, this runs with a peak airspeed of a very conservative 8 metres/sec.
A single 7.5 inch port would have run at around 11 m/sec (although it would have needed to be about 1060mm long)
If you flare both ends, I would guess that you could run up to a pair of twelves with about 300w total.
Look for drivers with an Xmax of around 10mm, which will be happy in about 80-150litres each.
regards
Collo