Rapid Fire sucks. The only time you need it is when bossing. When training, your ship rarely breaks (right now my 16k HP ship lasts ~10 minutes at Newties), and when it does, I find myself just using Air Strike more often. You still have Burst Fire, which is in many ways better than Rapid Fire. It uses less bullets, it can be used while jumping (great if you need to step back a bit while shooting - you can dismount, jump back and Burst Fire, mount while in midair and begin shooting again - I dunno if Rapid Fire has a similar application), it has higher KB, and it DOES MORE DAMAGE until you have 5 Rapid Fire). If you're doing a bossing-oriented build, by all means, but if you want to improve your training experience, you're better off getting more Torpedo and Air Strike.
Cannon is your main attack. Max it first.
Cannon is your main attack. Max it first.