are replacement iPads new or refurbished?

Agree with King_Penquin.

They can be either refurbs or new depending on what a particular Apple store has on hand.

Apple refurbished their own products and a warantied as same as new in both quality, condition, performance and reliability.

Either way, you get a good product.

Could it either, replacements 'will be new or equivalent to new in both performance and reliability'

If you don't have an active AppleCare+ or AppleCare Protection Plan, it doesn't really matter, as you are out of warranty at "under" two years.

Having said that, replacement devices are likely less a risk than a new device as they were individually tested and came with a new battery.