Out of the main exhaust manufactures Magnex and Mongoose (which Im sure are slightly rebranded Magnex's) have quite big rear boxes which on a lowered car 40mm+ could (COULD not will) get caught on things where as Scropion/Janspeed/Piper have more compact rear siliencers.
If I was going for a Piper Id go for the 4'slash cut as it looks spot on and sounds even better than it looks. 4' might sound too big but with the slash cut its nice.
Try either
www.performancecentre.co.uk ,
www.motorsportworld.co.uk or
www.speedbox.co.uk for best prices on Pipers