Is it OK to test diaper performance using regular tap water and table salt?

It is not OK.   Tap water has a variable content of minerals, specially calcium and magnesium, among a few others.  It is quite possible that the salt content on a given day of the year could be different from salt content on another day. Because these minerals have an effect on the performance of the SAP content in the diaper core, it is much better to use 100% distilled water, unless you can be absolutely sure that the water hardness has not changed.  pH variations in tap water also have an adverse effect on the performance of the diaper.   In order to reduce variations associated with the use of iodine salt, we recommend  use of 99.9% chemically pure NaCl available in most chemical stores.  If you use the same brand of iodized table salt, the expected variation is actually very small but is up to you and the precision required by your testing procedure.