Ah.. 400dpi is alright.. but I don't think "THAT" low is necessary.. since it is scaled from 800dpi.
They also cheaped out on the RUBBER grip on the G400 compared to the old mx510 and mx518, the new grip is FAR LESS grippy, and harder to pick up than the old grip.
I just modded my Mouse to FULL Omron.. Big difference... Everything feels as it should..
note, if you don't have a "solder sucker" don't attempt to "desolder" the switch, the traces are very thin and can easily be burned through. JUST CUT THE SWITCH OUT.. with an exacto knife...
IF YOU HAVE a solder sucker, you're likely pretty experienced, so this should be straight forward.