Answering your questions..
The absolute max limit is set by the programming of the board, not the capability of the Fusion. The current board caps the Fusion at 25 bps.
All Fusions sold now have the latest board, which is characterized by having 4 dipswitches and 8 modes of fire.
There are several items available now for customizing your Fusion, most recently a 10 piece 'Hyper' color kit. Many items are soon to follow, including a custom feed neck, grips, and trigger.
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if there are further specific questions.
In our experience, the best way to decide the old question of 'which gun to buy' is to try them yourself- it's in our opinion the best way to decide.
Hope this helps!