Joe,
i would personally be looking for other issues with the brakes for being ' spongy ' as you put it
even standard brakes are firm and stop the car well, but like any car need looking after, this goes all the way from new pads and discs to changing out the entire fluid every two years. as recommended by jaguar service sheets ,
on top of this you may have a little air in the system which makes it spongy, or even have the start of the main brake pistons failing,
the brakes could be ' glazed ' making the friction surfaces slip ,
like i said, even standard is good, dont waste money by putting enhanced systems on until you get your existing system working 100% ,
BTW these enhanced systems are not required for every day driving, they only come into their own when going like an idiot round country lanes where the brakes heat up a lot, or on the racing track
its just a way of making you part with your money to feel like you have something ' better ' on the car
and remember, its the only system you have to stop you ploughing into something in front of you, get it sorted asap
BB