true duals with X pipes in are still "true" duals the x pipe just allows both banks to scavange gases from each other and balance the engine. even ones without often have H pipes in them to allow both banks to balance. this makes a big difference and can help maintain torque instead of losing it.
as for what to go for, have a chat with Tarmac or go visit his website at Tarmacsports
i'd go high flow over de cats, de cats will allow more power but will add a rasp to the exhaust note (its personal choice, some like it, but i find it a little de-valuing on the Zed)
for about £1k you should be able to get a nice setup.