Ben, good morning,
my trade has many participants who will quote you whatever figures you want to hear just to separate you from your money.
I would look deeply into just who is offering you these outputs for £300. MTE, who I represent here in the UK are renowned for writing software that gives you the very best outputs for rapid progress on the road combined with a knowledge of just what any given engine will take in terms its longevity, emmissions etc.
Just search, both here and at
www.t5d5.org in depth for your intended supplier and check the provenence of what you are intending to purchase.....
Best regards, Don.