I'd still say keep weapons platforms separate from carrier platforms.
The thing is, Carrier platforms will have SOME heavy weapons, mostly for close quarters weight, not long range or strength. Well, close quarters weight AND breaching ability.
i.e. 50 gazillion hellguns and a couple of hand drills should do eh?
Same in 40k, other games, or anything else. The philosophy is to keep different aspects, well, different.
Why? one word. Efficiency.
"But having one uber unit instead of many different units is efficient isn't it?"
Well yes and no. If it's good enough to put all its aspects to use, and still survive, it's inefficient and unrealistic due to material cost and difficulty of construction. (Places where this doesn't apply: See Imperator Titan)
If it's NOT good enough to survive, then when you lose it you not only lose all the troops its carrying, but a tremendous amount of firepower as well. Hence why you keep things separate.
Also, losing guns or carrying capacity allows it to be smaller, lighter, and so you can take more of them
Now on to the actual rules:
Back Sponsons - solid idea. Always good to have something to cover your arse especially if you're a giant tank... something many other vehicles in 40k seem to be lacking. *awaits the wrath of the many-a baneblade fanboys on this forum*
2x Lascannons on sponsons: To clarify, these are twin linked lascannons or just two?
How many structure points will it have?
Will it have an assault ramp type thing, and if so what are your planned rules?
(IIRC Vanquisher cannons are used on Russes right? If so is the vanquisher option for main weapon a little under powered?)
Consider dropping some big guns.
Think about
: Open topped, firing slits, disembarking areas.
And lastly, 900 points plus 40pts/back sponson man? I can get a warhound that will blow this thing out of the water (or, if on land, into the water then out again) for a measely 750 points, so still trying to decide whether this is OP or UP...
In any case, a Baneblade / Gorgon hybrid is definitely a cool idea, so cookie points to you sir!
cheers,
Torq
P.S. what's the carrying capacity for a Gorgon? would be handy to work off that.
EDIT: ignore the demolisher comment (deleted) I just reread your post and realised that they're hull mounted. Sorry.