Worth reading an old stickied thread:
https://www.themgzs.co.uk/vb/showthread.php?t=30
If you are handy with tools and knocking up brackets then go custom to save money. I’ve had my BMC CDA fitted for around 14 years give or take. Same gains as the ITG, prior to being the first KV6 to be mapped it ran 192bhp with the BMC, decat and Janspeed system. So to me that was a healthy starting point.