should I use it all on thunder?

[Dr. Zeppers]

Lightning although one of the most effective cannon shots, is also one of the slowest reload/fire rates allowing only 1 shot per 'round' (second only to explosives in my experience).

Not sure at what level it becomes available, but I found the Rapid Reload skill to be quite helpful before I pumped up lightning. The primary skill improving firing rate is also helpful.

To each his own.