165/60/12in wheels have a 19.79 diameter. I'm not sure what the 145 10s have but the MPH diff at 3000rpm is 51 for 12s 50 for 10s so I doubt the highway speed gearing would affect fuel economy that much. The 1755013s that I have give 52mph at 3000rpm with that same final drive and I know that they slow the car down a lot during acceleration, they felt like dead weight on my 998.
I'm going to caution again against all the grill block advice because of the known cooling problems that minis have. Yes this is a great hypermilling mod but the mini cooling system is horrible. For those who don't know it was one of the first successfull transverse engine cars so the engineers just used the same style rad that the old front to back engines had and used a pusher fan instead of a puller. I've read somewhere that the pusher style is much less efficient. Also the engineers found that the proper fan was too loud so they chose to use a multi blade design that was not really up to the job. There is a tropical spec fan (which I have and is much better) that they had to put on to sell the car in warm climates. On a hot day a mini can overheat just idling with the UK spec fan. If you do block it do only the right hand side and also direct the open part towards the radiator
As for the ignition it's really easy to check on the mini, all you need is to remove the front grill then loosen the nut that holds the distributor tight and turn it slowly towards the rotation to advance, away from it to retard. You'll also need someone to watch the tach or a tach you can watch from outside the car. Here's a great article telling you how to do it
Mini Spares - Article - Distributor - Initial ignition timing set-up.