It's also worth mentioning that the 350C upper limit of the U5855A is already well above the entry level of most comparably priced units (not list price, but what people are paying secondhand), so there's sort of less of a need for such an upgrade. Still may be worth trying, and it seems like many of the units Keysight is sending out through their ebay store are still on the 1.00 firmware.
I got mine a couple days ago, and while I was happy with my first look at it, the charger I was expecting to fit ended up being a 2.1mm center pin, so I'm waiting for a new one that will fit and the battery has died. Though it was corrected earlier, it's worth mentioning that you need a 2.5mm center pin in your barrel jack - something I didn't even have a spare connector to fit!
In my limited time, it seemed to be quite sensitive and auto-ranged pretty well, but some of the color palates appear noisier than others because of how much the colors scale - I think I'd prefer something like Flir's red/blue combination mode, though for instances when the palate appears noisy, grayscale or iron palates work well. The manual focus seems to work fine, but you have to tweak it more at distance - your in-focus working distance within half a meter or so seems to be pretty wide and you don't have to adjust almost at all, whereas farther targets take more lining up. It's useful to turn on the visible camera to help line up the focus, because when the images look properly overlaid and centered, you're in focus.
I'm new to thermography, but it's been pretty intuitive to use. I find that a lot of reflective surfaces aren't as misleading as they may seem - sure it doesn't read the right temperature, but it also has the same glare and sheen as a normal glossy or reflective object, so it's usually easy to tell that it's not the right thing, even in thermal-only mode. It also helps that anything that's really generating heat also warms up the stuff around it, so even if the finish is high polish, the stuff it's mounted on will give it a nice warm halo to give you an idea of what's really going on. I look forward to charging it up and taking a look at the house, inside some more equipment, and down to the park to see if I can spot any critters i can't usually.