As far as I can tell, 5-string players are happy enough with bass cabs designed for use with standard 4-string basses. However, the beauty of doing it yourself is that you can tune the cab to your own specific needs. I don't think you will actually hear any difference between a 40Hz and a 50Hz tuning, as the rolloff is an octave above the tuning frequency (unlike 6v6's Beyma cab) and the small change in frequency response is therefore unlikely to be audible.
There will be a difference in power handling though. The only problem, which you obviously are aware of, is that you will have to sacrifice some power handling at around 80Hz to get extra power handling at 30Hz. Personally, I'd stick with 50Hz, especially as you don't use the lowest notes of your 5-string much, but it could be worth comparing the power handling of the cab with a 40Hz tuning. Also worth bearing in mind is that a higher tuning frequency gets you a bigger port area, which is quite important with a cab this size.