Of course is cost is no issue, get a Lobo light package -- there is no equal in quality, elegance, reliablity or efficiency; however....

If you want to save battery power -- then, in order of preference, I'd replace the ANCHOR LIGHT first (since you're likely to be using that for the greatest amount of time with the least supplementary charging available.); then,

I'd replace the sailing running lights (if you have a masthead tri-color, then only that); and, only then,

Would I replace the steaming light on the front of the mast.

I have Aqua Signal lights all around. I've anchored out often, done a good deal of cruising (four trips to and from Southern New England, plus Chesapeake Bay trips), and a bit of night sailing. I haven't replaced lamps except the bow lights; and there, it was improper sealing of the wire leads into the back of the lamps that leaked.

As for the USCG issues: As other mention, if you get into a legal squable after a collision, then you'll regret not have USCG certification. The issue of brightness is complicated only one of the issues, there are issues of reliablity of color as i) the bulb ages, ii) different temperature challenges, and iii) how you measure percieved brightness across the spectrum of visible light. (Micheal H may weigh-in here.)

My advice, if you care, then go certified. I wouldn't consider otherwise in the U.S. Standards for such things have a purpose. WHY putz around with this. If it's an interior light or something supplemental, then do whatever you want. Do you want the other boats you're trying to avoid to be "improvising" their view of adequate lighting?