I'm big on rule of thumbs for things. I have one for range at typical e bike speeds, and another for sag. I think sag over 6v is bad.
If full throttle from a stop gets you only 2v of sag, that is outstanding. This will likely mean less than 1 v sag at cruise. Its typical for a bike using RC batteries of at least 10 ah size, and 30c (claimed) c rate. To get this, you are actually pulling 5c or less usually, with ordinary cheap hobby king lipos. So a 1000w setup, doesn't hardly strain this type battery at all. Up the wattage ante enough though, and you can be back to high sag on takeoff. That is where you are, that sag is costing you acceleration. A larger pack will sag less, but it may still not get your wheel up. A more poorly balanced bike will get the wheel up at 500w though. So you might look a bit at putting that additional battery farther back.
Other types of batteries might be 3c claimed, or at least not much more. This is typical for e bikes in the sub 1000w category.
In this case, more sag is definitely typical. IMO, if the sag is more than 6v, then you tend to have a battery that gets very noticeably hot, not just toasty warm by end of ride, and this is not acceptable to me. It might be ok to the industry, who's only goal is to make that sale. To make that sale, they undersize the expensive part, the battery. They know it will last the warranty period, but not really last. So they will gleefully sell you a system with 8v sag or more. Bear in mind, cruise sag may be ok, but start up sag not.
4 or 5v sag is ok, so size your battery so that your motor system does not produce much more than that, or, ride such that you only see that 4v sag. Feather the throttle on take off, then give it full power once moving 10 mph. This will help your battery keep its cool, and last a lot longer. Ideally of course, you pack enough battery to get 4v sag or less no matter what. But it may not be all that practical to carry 30 ah to get this. One option is to parallel your low c rate 18650 cell pack with some higher c rate lipo, when you will need to pull the bigger amps spikes. Like mountain riding. Then you get the best of both worlds, without going to a 1500 watt hour battery.