Roger
You’re asking for help, everyone that is quick is saying use a one way diff and no-body has any issues with using it. So it’s either don’t buy one and have an apparently slow car that doesn’t suit you, or buy a one way and have a fast car that doesn’t suit you!
You’re right about the cost, but everyone I race know and race with (not just F1’s now but all the lads I know) uses one in a range of cars (XX4, X5 and BJ4’s primarily though). My shiney front CVD’s have been in since the indoor finals so have done a whole season and though a little worn, they won’t be changed anytime soon.