Is the 9000FT worth it over the 8000FT?

In the same rack space, usually. The case, the footprint and the mounting are identical, so the only physical cost is 9.26 pounds a block. For that you gain 86 watts per cell at 15 minutes, 5 amp hours, and a drop in internal resistance from 2.17 to 1.86 milliohms. If the rack and the shelf rating already take the 8000FT, there is no fit argument against the larger block and the decision comes down to budget.

What does 1.86 milliohms mean on a real string?

On a forty block bank that is roughly 74 milliohms of internal resistance across the string, against about 87 for the same bank built on 8000FTs. Under a hard inverter step the lower figure means measurably less sag, so the bank stays above its cutoff voltage longer and delivers more of the runtime it was sized for. On short strings the difference is small; on long ones it compounds.

What does the 9000 in the name represent?

A nominal output for the whole block at the 5 minute rate. Six cells at 1,403 watts come to 8,420 watts, which CSB rounds up to 9000. That is the largest gap between badge and measurement anywhere in the range, about 6 percent, so a string sized from model numbers rather than per-cell figures will come out short here more than on any other XPL model.

How should a bank this size be maintained?

Take an impedance reading on every block at commissioning and record it against the serial, then re-read on a fixed interval and watch the drift rather than the absolute value. Front terminals mean those readings happen without unracking anything. Keep float in the 13.5 to 13.8 VDC per battery band, and log the room temperature on the same schedule, because ambient is what consumes design life on a bank that is rarely called on.