Imho the current rules for torpedo salvos are a solid abstraction that plays well.
They remind me of the common WWII tactic of firing torpedoes in spreads to increase the likelihood of a hit against distant targets. Firing more torpedoes not only increases potential damage but also covers more area.
Reducing torpedo salvos to a fixed size will completely remove this advantage and weaken torpedoes in general in BFG. Just look at the discussion over the CWE Dragonships weapon options:
6 torpedo tubes is nothing compared to 4 launch bays and with this 'fix' the torpedo option will become even less appealing, despite its increase to 8.
Also I did some experimenting with 3D torpedo markers but gave up on it as its not practical. Torpedoes unlike other ordnance ist not removed on first contact and often has to be placed in a space which is already occupied.
The fact that torpedo markers are flat makes this a lot easier.
Think of a group of bunched up ships where you have to place a torpedo marker in the middle. And now you have to put a die on top of it?
Nah, I don't like it at all.

Edit:
I totally forgot to mention that size
does matter.

One of BFG's perks is the fact that even innocent bystanders can get a quick grasp of what's going on. Blast markers show were the action is and the threat of a salvo is plain obvious.