I have bought it (POR15, Marine Clean, and Metal Ready)from their website and used it several times for various applications, including coating my entire tub before installing insulation and new carpet, similar to what you are thinking. I was happy with the results, as my tub was in pretty bad shape. A quart did two coats easily and really protects the metal, if you take the time to prep it right.
That being said, I think that you might not really need it on the entire tub...it looks like your paint is in good shape, except for the footwells, as you said. In order to really seal and protect the metal, POR15 recommends that it be applied to bare metal, properly prepared. POR15 does not take well to glossy paint, therfore (IMHO) you would be wasting a lot of time and effort doing the entire tub with it. It might look better, but I'm not sure it will matter in the long run. I would spot treat the problem areas with POR15 (using the Marine clean and Metal Prep) and take the time to do it right.
I also used POR15 on the suspension parts, the battery cases and parts of the underside of the tub that were bare metal, again with proper cleaning and prep, and am very please with the results. You must top coat any parts that are going to be exposed to sunlight, as POR15 is UV sensitve and will discolor from direct sunlight.
Oh yea...use the gloves, too...any POR15 that gets on your hands (or anything else) will be there until it wears off...in a couple of weeks!
Just my two cents worth...which I beleive you asked for?
Bruce