Let me try to sum it up:
"Broken" Demo build: Glad-I, Engine Techs, Intel Officer, Ord Experts, Exp Launchers, Demolisher (56+8+7+4+13+10=98 pts), under the command of Screed. First player, five activations.
(you can of course do and ACM variety, but then you get slightly less utility from both Ordnance Experts and Intel Officer, so its not quite the same thing)
Taken in isolation all the upgrades are perfectly fine. Its the combination, and especially the shoot-after-move ability of Demolisher, combined with the other upgrades that make it problematic.
Turn X: You jump in (possibly as the very last ship to move given your high activation count), preferably from BEYOND red range (sorry Ackbar), using pseudo-speed 4. Which in this case is even better than true speed 4, because it makes it insanely easy to set up that double arc for next turn. Using your 2 red, 4 black, with re-rolls and Screen, you Intel his brace (only Nebs have double braces, but they have such vulnerable side arcs, so Intel isn't needed). If he braces he might "only" be taking a fistful of damage, but then the brace is gone. If he doesn't brace, its a huge amount of damage.
Turn X+1: Double arc. If the brace isn't gone yet, it gets Inteled again...which essentially means only 1 brace against 3 attacks. 2 red + 4 black, then 4 black. With re-rolls and Screed (some players might even sneak in a CF here).
The only things that can survive and attack like that is the ISD and the MC80. But even they do not always survive. I've killed a completely fresh MC80 with a triple-tap (ACM variety). Granted, my dice were hot, but it goes to show how mighty this combination is.
To compare: The closest thing you can get is an MC30, kitted out the same way (sans the Engine Techs). You could even toss on XI7s or TRCs (Scout only) for further shenanigans. But even a Trop MC30 with full kit and XI7s don't get that triple tap. And that's a HUGE, HUGE difference. A double trap simply cannot reliably kill tough prey. A triple tap can. Furthermore, while the MC30 has native speed 4, it's nowhere near as easy to set up for double-arcing anything small.
Can the Demo be defended against? Sure. If you make a good list for that purpose, AND you play well. But generally speaking, you cannot prevent a 1st player Demolisher from jumping in and killing whatever it wants to kill. All you can do is to set up a retaliation strike, to either take out Demo or to punish his other ships for the triple-tap.
So is Demo OP? Given it's power, it's ease of use, and how hard it is to deal with, I think it is.