HiMAT stands for High Mobility Aerial Tictacs errr... Tacits errr... Tactics!
It is allows Freedom, Justice, SF and IJ to track multiple targets and aim each of mobile suit weapons at one designeted target. Sort of aimbot really. Well, jokes aside - it is multitarget tracking and aiming system.
Also works when aforementioned mobile suits are docked with METEOR system. HiMAT has nothing to do with suits propulsion. Nuclear-powered Gundams are faster because you can devote more power to thrusters without worrying that you will run out of power soon. Destiny can move fast using it's WoL propulsion system, but it drains Destiny's hybrid reactor's power faster than suit's reactor can replenish it. And seems, Destiny and Leged can't use Minerva's Deyterion beam to charge suit remotely. Destiny is faster than SF in raw speed but only for a short time. Compare Destiny to cheetah - it can run very fast but for short distances. Useful only for charge-in type attacks. Once your attack misses you have to turn off ultra-speed or you will find yourself running out of energy.
It is pointless to use HiMAT mode against ONE mobile suit because system won't allow you to aim all weapons at one target - you can easily do it manually, but that will make one concentrated beam blast which Shinn will block or dodge. Only way for Shinn to kill Freedom is:
1. Gang up on SF with all 3 of Minerva's Gundams, but they should also take IJ into consideration this time it will be alot harder than first time Shinn took out Freedom.
2. Use tactics!!! Again - twice as hard now with upgraded Freedom and IJ covering SFs back...
SFs wings are just DRAGOON bits - they also have nothing to do with propulsion, they are in essence additional beam rifles (i think their power are comparable to beam rifle) which can be separated from mobile suit and controlled remotely to attack designated targets or used with HiMAT system for multi-target blast.