I get your point afterimagedan. As you say, the original Tyrant is overpriced, but I just think the simplest solution, and most stable in terms of internal balance for the list is to tweak the points cost and not tamper with the stats.
In your second paragraph there you're sort of saying 45cm WBs shouldn't need to be taken, and if people are using them, they're sort of playing Imperial Navy 'wrong'. That seems like an argument not just against the base Tyrant, but also the up-gunned one (and indeed some Battlecruisers like the Overlord and Armageddon). Anyway, I think we don't need to argue the case for/against 45cm batteries on a line cruiser. It's not really what we're talking about... What I'm really arguing against is a 12WB 30cm Tyrant, for reasons you know already.
Anyway, me just saying I don't like it isn't helpful, so I'll try to give some alternatives:
1) Make the up-gunned version the only version! This is the way most people were playing their Tyrants pre-BFG:R anyway. It also gets a little points drop:
10WBs at 45cm L+R, 6 torps F, 190pts (+20 for NC).
2) The original version, but with a slight tweak:
6WBs at 45cm L+R, 4WBs at 30cm L+R, 6 torps F, 185pts (+5 to boost 4WBs from 30cm to 45cm, +20 for NC).
3) The original version with a drop in points:
6WBs at 30cm L+R, 4WBs at 45cm L+R, 6 torps F, 180pts (+10 to boost 30cm WBs to 45cm, +20 for NC).
I don't actually like all of these proposals myself, I just want to get a proper discussion rolling, and I am ready to compromise on some things. Also, they might still seem a tiny bit over-costed to some. Anyway, that's something we can discuss.