I can't take credit for that dome-top. It's a standard PlayPac feature.
Upsides: More light
during the day and j-u-s-t enough added headroom that it let me stand up straight if I stood in just the right place.
Downsides: Requires extensive attention or else it leaks
like crazy! Mine came with half a dozen layers of various gunk layered into the seam trying to stop it. It's also hard to fit a shade onto. To put it simply: I'm not a morning person. Having all that extra, unobstructed light
streaming in in the morning wasn't all that welcome.
Similar domes are fairly easily available (here's a really big one
The main problem you'd have is getting it to fit on the non-flat roof correctly. You'd presumably run it down the center of the trailer to give room in the aisle.
Another - much more intensive - solution would be to crack the trailer apart at the seam - add 6 inches or so into the gap - then put it back together again. Not something that could remotely be called a 'small upgrade', but gives you headroom everywhere. Course, it'll also screw up the side closet fit (not THAT hard to deal with) and the fit of the door (PITA problem).
I suppose a sort of combination of the two is possible too. slicing off the center bulge, adding a spacer and reattaching. Just as big a hassle as doing it at the seam - but you don't have the extra problems (closet & door).
Course - I don't personally think that having tons of headroom is all that important. 95%+ of the time in the trailer is spent either laying down or sitting. I don't cook in the trailer (prefer doing that outside) and we're not dancing in the trailer either.
The ceiling height on the cargo conversion I'm supposed to be working on is REALLY low (something like 4.5 - 5 feet inside). I briefly toyed with the idea of adding a dome onto the top for standing room then decided not to bother. My little fiberglass cargo trailer has a flat roof too - so it would be relatively simple. I just decided that when all was said and done, it wasn't that important to me. It's tall enough (barely) that I can move around on my feet (beats a teardrop in that regard) and the extra height wouldn't be used that much.
The biggest appeal of the pop-tops IMHO is not so much the headroom - but the idea that you could replace the fabric sides with nylon window screen material for the ultimate roof vent. That would be my primary reason for considering such a thing.