Re: Replacing the batteries.
Are you high? Engines degrade over time with wear and tear on the drive train, on the valves, maintaining compression etc. Do you really think that when you stick a 10-20yr old car* on a dyno it will generate the same bhp?
It depends on how well its looked after. When I had my play thing tuned on a rolling road , it'd lost half a pony from book value in 15 years (0.5 bhp drop). In the interests of full disclosure, its given a full service annually and an oil change every 6k or after winter whatever comes first.
EFI and ignition coupled with proper exhaust gas analysis mean coking doesn't happen any more, and wear & tear can be minimised with a proper warm up and cool down procedure and frequent oil & filter changes. Modern oils are a massive help too.
Don't mistake this post for concern that a Tesla loses 10% of range over 160k miles - that sort of range drop doesn't bother me. As I've posted before, my bladder can only manage about 200 miles between stops, so anything with a greater range is mostly workable for me these days.
 its normal to do a before and after run if you're having a lot of work done so you can validate the improvement and ensure any problem isn't caused by the changes.