I guess the question though is: if you filed the N400 9 months ago, and are just now about to combo interview, did it really help? Or would your I-751 be approved by now anyway?
The cases where it seems to have definitively helped are like those of @Crazy Cat, who had radio silence for 44 months, and then a combo interview almost immediately after filing.
Our LIN filed I-751 was approved 18 months after filing, and the USCIS processing time for that center is 20 months. N-400 processing time for Seattle FO is 9 months. Whenever the husband gets round to filing it, I’m expecting it to be adjudicated within that timeframe. We’re in no particular hurry.