Why only 15? the 737 (your type iirc) can do up to 20 on a wet runway and 25 on a dry one
Actually you can even get approval to land with down to 125m RVR if you have a LAND2 indication, 50ft RA of course but lower than usual CAT 3a RVR.
Having flown a 733 with HUD manually to the usual CAT IIIa limits all i can say is that manual landings are possible and actually quite a lot of fun, given the right equipment. However with any failure of equipment close to DH or below i would personally rather do a go around then try to sort things out in the very short time i have until landing. But there is allways a somewhat grey area, do you initiate a go-around if the rollout feature fails during high speed rollout in the fail operational 737s? Even if you have allready the reverser open? And those cases are what captains decision is there fore, that is why that position is usually better payed than the first officer who is not allowed to fly actual low vis approaches (simulated ones are fine though).