Well your 512 timing thread did take a left turn into fuel milage and cruise circuit tuning but I was not talking about fuel. (Although I never heard of changing the transfer slot- why would you do that when you can just alter the mix going to it with the ifle feed restriction and air bleeds?)
Adding a restrictor to the transition slot passage gives you another tuning parameter. You can have the IAB and IFR right, but the sweet spot for the transition from the idle circuit to the main circuit is rich. I don't know about 4500s, but the 4150s I have with open transition slot passages are close to .120". Bringing that down to about .080" helps lean out that point so the transition is cleaner.
