I've gotta agree with DropFall on this, I wouldn't make it any larger than the Battlecannon on the Leman Russ. There is a twofold reason, 1: we have a basis of comparison as they are very similar stat-wise, 2 you can easily trade for one as the standard Leman Russ pattern isn't modeled too often (well it doesn't seem to out here, see a lot of the Exterminator, Nova and Punisher variety, but this may be one of the oddities of my area).
If you want to go bigger then you run the 'risk' of people misinterpreting and believing you are using the Forgeworld rules for a Supakannon Battlewagon (which is a most impressive weapon, but not worth it in my opinion).
If you realy have to add in big shootas, just add one. I prefere the grot turrent that comes with the kit, but the spotter commander kit works good too.
The reason to only take one is so you are wasting the least amount of points. If your KK gets blown off, then the next shot should get your BS, either way having a weapon makes it harder for your opponent to get a destroyed result on your BW
Skeetergod in your post you state that more than one Big Shoota is a waste, and frankly I have to disagree. I usually use 2-3 Big Shootas on my non-transport Battlewagons (and switch from Kannon to KillKannon), and I find that the Dakka helps. Yes you have to be stationary to let everything rip, and while you are on the move it may seem a bit pointless, but the best reason for Big Shootas you've already pointed out, they are essentially wounds for your Wagon.
Now I can only speak from my experience, and in that experience I have noticed that I end up on the receiving end of a lot of Weapon Destroyed Results (to be honest I'd say a disproportional amount of Weapon Destroyed results). Lets say for example you have a Wagon with just a Killkannon and your opponent gets 3 Weapon Destroyed results in a row (it happens!). First your tank looses its gun, then its Immobilized, then its wrecked. Your Wagon is officially dead. But if you happened to have the spare points (a couple cheaper then 2 Ork Boyz) to grab 2 Big Shootas that Wagon could at least still Tankshock and Ram (and bulldoze if it has a Deffrolla).
Now I admit fully that it is my jammy luck to receive a lot of Weapon Destroyed rolls, and as such my opinion is tainted. It's a 1 in 6 chance, and the laws of averages say that you'll receive just as many Weapon Destroyed as Shaken or Stunned (or the other 3). But as I already said if you have the points lying about for a couple Big Shootas, and you already have your bases covered with maxed Boyz units, I can't think of a better expenditure of points. Well maybe Ammo Runts for a SAG Mek or Flash Gitz, but I think they're worth the points.
Peace
~OD