If you already have a 10.5, I would stick with that and turn the timing off from the speedo. A non -boosted 10.5 is not that fast at all, but you benefit from increased 'feel'. A boosted 10.5 is almost 'mod' speed but the 'feel' is rubbish because of all the added timing and turbo. You can always add speed via the timing feature on your speedo once happy with how the car is handling. Cheers!