If you insist on getting a can, at least get a Triumph Off Road with their download. They give a decent performance gain without being obnoxiously loud. To be honest I havn't ridden one with or without a TOR, but I do have experiance with other Triumphs and TOR's and they are a good compromise twixt stock and neighbors-throwing-rocks-loud..
If you're just looking for a little more power, try intake rather than exhaust. A power commander ECU set up with a dyno tune will give you what you want without extra noise (I think the 675 sounds great with stock exhaust). It will also smooth your horsepower and torque curves. Do a dyno run before and after-you will see the difference (they'll give you a printout)!
I've ridden and dyno'd it both ways see our 2006 Middleweight Shootout and I personally liked the stock pipe over the TOR system. I think this is because the TOR system gives the bike a "deeper" tone and the cool thing about a triple is the way it wails and howls. When you make the tone deeper it sounds closer to an inline four, loosing some of its three-six-twelve cylinder wail in the process. Besides, the best noises come from the 675's intake anyway.
If you spend the $600.00 on a good riding class vs. the can, intake, download or combination of the 3, you will be mush faster no matter what bike you ride. It is a lifetime investment vs. a 2-3 year investment.
The new Jardine slip-on (actually, its a little more that a slip-on, since it esplaces the down pipe all the way to the header) will get you an additonal 5hp on the stock tune. Likely more if you use a power commander or tune boy and a little dyno work. All other slip-ons available for the 675 that I am aware of will work fine with the stock or TOR tune. Just make sure to avoid the Arrow tune with the slip-on, as that will not work at all.
Finally someone who agreees with me. If yo are riding on the street, youhave way more perfomrance than is sane. Take a riding class and you will live to ride longer. Thanks to Freddie and Nick for teaching me how to control my bike better. I needed it this morining when a cell-phone-talking-coffee-drinking-SUV-driving woman cut across my bow with only about 4 inches to spare. I was on the brakes for all it was worth. Thanks to Freddine and Nick I know the life- saving difference between stab and grab and squeeze quickly.