Hi Simon,
Sorry for the delay replying. My opinion is save a little and get both mate, starting with closed loop. yes, we can get it almost as good without it, but its not easy, and the slightest thing changes on your engine and it will be rich/lean. Thats the beauty of closed loop, it constantly changes to keep your mixture correct and can compensate for sensor drift and inlet depression changes with no problem at all. Running at Stoich is also great for engine logevity and oil consumption as we have far less fuel left unburnt and thus less making its way into your oil.

Our closed loop has had most of our development time devoted to it too, so as a rule you will experience a better cleaner performing engine with it as opposed to without it as at least 90% of the live maps we have done in the past 3 years have had it active in the chip.

Final point... every single car on the planet manufactured since 1994 (And a LOT before then) has had closed loop as standard. Catalyst requirements aside, Its the way forward for good emissions and fuel consumption.