You are right; the A/F is extreamly high under coast and downshifting(which is why your gauge blanks out), but that is how its supposed to be. Your car should not be giving the engine much fuel when you are not giving throttle input and the engine is already spinning faster than idle; example- rolling downhill in gear. Fuel is used to speed the engine up, not down. What is the air fuel ratio under boost? If it is safe than the tune is just fine.