ptrefry
Legalities of even having "live" side pipes aside, here's my 2 bob on side pipes;
As mentioned in recent threads, the noise can eventualy wear you down and the original style only affords about 600 mm of muffler, I've got just over 800mm and have trouble keeping the noise reasonable.
Everyone I know who has tried the original style have disconnected them due to noise and burning thier mates, the wife, the kids and even the dog.
By all means run them, but unless you are building and exact replica, most of these issues can be addressed by building them sensibly, suitably shielded, and they will still look good.
Every thing about a Cobra is oversized, wheels, tyres, engine, why not oversize the pipes to achieve this. Robnell's run a 5" pipe with a shield over that, and they don't look out of place.