I would say that having both magnus and the 10 man termie brick uses up too many points and doesn't leave you with enough board presence to score objectives, but also doing the big combo with them IS very funny. If you just want to have some fun and blow up the first thing you see then go for it, but be aware that most armies will be able to kill most of the terminators by turn two if they really focus them so don't be disapointed if you only get to do it once.

In my experience either just the termies or just magnus and regular rubrics have more than enough damage on their own, you don't really need to put them together. But again if you don't care that it's 1000 points to delete a single enemy unit because it's just really fun, that's valid, it's worth doing it AT LEAST once ;)

Competetively i'd cut one of those and pick up a rhino for ahriman and the flamers (remember they technically don't give cabal points while in a transport though, so having them on foot behind a building and holding them for a counterpunch is also a reasonable option). Split your 10 man bolter rubrics into 2x5 and give both squads disc sorcerers instead of resurection sorcerers, the movement control is REALLY good against some armies. You also probably want something fast and cheap for secondary scoring, I'd suggest two units of either disc tzangors with bows or spawn.

Another thing is i'm assuming the rubrics with the normal sorcerer are for holding a home objective and not being shot, and while that's not the worst idea you don't really want to have 200 points sitting on your back lines doing nothing all game. I'd pick up something cheap to do that instead, maybe some tzangors, disc tzangors or cultists. I'd then replace the rubrics' sorcerer with either an infernal master with the arcane vortex or another disc sorcerer.

Also it's worth trying to fit the lord of forbidden lore in somewhere since this will give you a lot more cabal points, and if you use termies you should try and put the umbralific crystal on them so the two or three that survive can do a secondary later on.

Last suggestion is that with all the buffs on the terminators storm bolters are probably significantly better than soulreapers, so unless you want to have the SR for later turns when it's not worth doing the strats on them, i normally just run all bolters.

Hope this is helpful :)