Hmmm - again so if you are taking off on a LONGER runway, you have to re-run numbers again, to use possibly-more-detuned performance rather than just keeping specs you know will safely lift you off, or using full power knowing that you are "bounded" within safety?

Obviously you don't want people being too creative as it can lead to unforseen problems, but you do a complete recalculation with a small change in info? It seems that running the possibly rushed calculation would lead to more risk than using some logical sense, as you allude.