Gary before you go spending big bucks on front springs, may I suggest this, take the weight of the front end, centre stand and a jack under the front of the engine, undo the front fork caps, add some washers on top of the springs,I ussed 2x 20cent pieces in each fork, this will help preload your springs up and this makes a huge difference. I did this as one of the very first mods. 20% stiffer on the front is a hell of a big step. I would worried that such a big step would then make the front to stiff, and in fact be very uncomfortable on the great, and smooth roads we have.
In fact I would worried that in fact, would be unable to absorb the bumps and have a tendency to want to throw one down the road under bump, brake and corner. First try the spacer trick as well as going up a grade in fork oil at the same time, 15 weight may be a bit over the top, but some shops will mix up a blend of 10w and 15w to give you something in between, me I was a bit of a big fella on the B6 so I was pretty happy with 15w. As for the back spring, there aint much to be done there except start again with a shock with more than just a preload adjustment.
I personally think the dual system, is not the go, unless the dia of the pipes to the mufflers is really really small and the mufflers are really really packed full of muffer pack, it will not supply enough backpressure to work as well as your stock system, or unless its long enough to be an undertail twin system, even having said that most twin under tail systems are feed from a single mid pipe. Go the single 4 inot 1 gain the hp and loss some weight. With the bucks saved over buying the dual, a really shyte hot 4 into 1 system with all the bells and whistles is well within. reach.
Now for some bang for ya bucks, ignition advance, which there is a zillion ways to do this, airbox mods, simple easy to do, and air filter ungrades.
Handling preformance mods some cheap ones are drop the forks through the triple trees, 5mm 10mm 15mm, im sure you'll find the one you like, or something in between. netx time you replace the front hoop, try instead of the normal 120/70/17 a 120/60/17 or even 120/50/17 this makes a huge quickening of the turn in, along with the rake change of forks through the clamps can make a slugish turn in, bike into a corner muncher.