I feel we've strayed a little off the point here, so I'll sum up the answer for you:
- Mosfet: At their most basic they are a "gate" that controls the flow of power to the motor by way of 1 or 2 additional signal wires. These are wired to the trigger contacts in place of the power wire(s). This allows for more efficient power transfer (resulting in better trigger response) and prevents carbon build up on the trigger contacts as they are no longer part of the power circuit.
More advanced fets add voltage protection, burst features, pre-cocking, active braking bla bla bla.
"Plug and play" mosfets will only give you burst functions and voltage protection as your trigger contacts will still be part of the power circuit.
A mosfet cannot improve your rate of fire in any real sense (they may be very slight improvement due to more efficient power delivery). To get any real gains in RoF you need to look at upgrading your motor and battery to start with and gears if you want to go further. This does lead to its own problems as you will be putting your gearbox under a lot more stress. So then you need to look at gear shimming, lubrication, angle of engagement, sorbo pads and so on (and on). They are hundreds of written and video guides on what these things do and how to properly do them.
Deans connectors: basically are a better electrical connector than tamiya style ones (they can handle a much higher current), will add to the efficiency of the power circuit. These are advisable to use if you use lipos.
Bottom line is that if you dont want to get into the mucky muck of your gearbox then find a reputable airsoft tech to quote you on the cost of upgrading to a high speed set up.