While the tar is an addictive tool to use, it is still just a tool. Adding flammability as an extra effect would make it less of a tool and more of a weapon. As of now, briefly flying through tar will cause a mess of trouble as a punishment for chasing a ship. Making the tar cause even more trouble by adding fire may make it a go-to strategy- if you take tar, you have to take the banshee, flare, or flamethrower (Or incendiary ammo, I suppose), as well, in order to maximize its effects. As of now, the tar acts like a defensive tool well enough- use it to disappear, or to shake off a pursuer. I believe that the tar should not be changed, so the pilot must rely on the ship's gunners to do the real damage, not simply use them as an ignition spark.
As for the fire spreading mechanic, that could be interesting. Fire, currently, seems underwhelming because stacks of fire are only problematic at higher amounts, and fire mostly relies on DoT. Giving fire some extra power would be nice... but the spreading mechanic doesn't seem to me like it would fit in with the game. If a fire's charges grew over time, that would, in my opinion, feel like it would belong.