does this 2.5ah battery outperform the standard 2ah unit? if so, how much??

The 2.5Ah battery will provide a bit more runtime than a 2.0Ah battery. Approximately 25% more than a 2.0Ah battery.

Is this listing for a 6 amp 40V (see picture) or 80V 2.5Ah (see heading)?

I missed that photo mess up! Looking down through the description and other toys they show are 80v. I got the Greenworks leaf blower GBA80250 80V 2.5Ah Lithium Ion Battery with a spare. The batters seems to be last longer between charges now that I have had it awhile Only issue with the blower was the tube kept coming off. Replace the wire tie with a Aero-Seal Clamps and it swings left to right with out coming off.

Can you explain why batteries are not properly marked for amps? on the 2a-144wh 80v=1.8amp, on 4a 288wh-3.6a

The reason is that the voltage of the battery is 72V nominal/80V max. Nominal is average use volts, max is extreme use volts (surges, start-up, stall prevention etc). For example, 72V x 2.0Ah = 144Wh which would be the capacity of the battery.

Will this fit in the old style drop in charger?

McGee, your question is too vague to answer. I would not know what an "old style drop charger" is. This battery has a charger made specifically for it by Greenworks. That is the only one you can use.