I bought a pair of Bullet 50 (not Ultra) on sale a few weeks ago, but I'm still on training wheels (salt, grit, etc still on our late winter roads here).
As I've posted in another thread here, the weight as advertised is always for the lightest version. Bullet 50s are stated at 1755g, Bullet 50 Ultras at 1590g, but that would be for the Bullet 50 USB, and Bulet 50 Ultra with CULT, respectively. My standard (steel bearing) Bullet 50s weigh in at 1795g.
When you look at the weight difference across the range of Bullet wheels, the difference between Standard and Ultra is 165 grams (for both 50mm and 80mm wheels), also with a greater difference for the rear wheels than the front wheels. There is a slight weight increase from brass-nickel rather than alloy nipples, but the biggest difference is in the steel versus aluminium axle. The axle material will not make any difference to the rotational weight of the wheel. Of course, it's also a lot less likely to break (I've busted a couple of Mavic alloy axles in my day).
I also note that the steel spokes have a slightly narrower aero section than those on my 2012 Zondas. Presumably that's the same across all versions of the Bullet.
Don't plan on riding these extensively for a few weeks, but I'll share my impressions when I do.