I would definately NOT add a bow thruster. The conventional wisdom on this topic has been extensively discussed before. Any thruster that is added for a RC sub will have greatest effect if mounted in the stern.
Before going to the hassle of adding a thruster - consider weight. I am advised by Matt Thor et.al, that weight is crucially important related to turning as mass momentum is higher / harder to overcome with higher weight on board. If you are using a sealed lead acid battery - you may wish to consider lower weight metal hydrides or a lower capacity sealed lead acid battery. The location of battery weight in the bow can also make turning sluggish.
Weight is crucial I am re-engineering my 72 scale Alfa, not a large boat, but one with very small rudders. Reducing the battery weight but half has improved the turning radius by about 30%.
Hope this helps.