I've got a standard system fitted with a K2 back box. It's my daily driver and it sounds nice, with no drone etc and sounds fine on the motorway etc. I know Japspeed gets slated for it's quality, but mine was fitted to the car when I bought it and has now been in my possession for 15 months and about 14,000 miles, through winter etc. It hasn't fallen to pieces yet, doesn't rattle and still polishes up nicely.
It is very slightly louder than a stock exhaust and does sound 'nice'. Personally I would like something a little louder, so combining it with HFC would be my next option, or just change it for a Cobra if/when I can afford to.
If I were in your position and I could afford either the HFC or K2, I'd maybe go for the HFC first. But it would depend on the condition of your OEN backbox. If it's looking past it's best and may fail in the forseeable future, go back box.
Also, I've got a brand new K2 backbox sat in the garage that is surplus to my requirements if you're interested