i'm a fan of impartial 3rd party BoP testing. its more expensive, but a better way to asses a cars ultimate potential. its easy for a team to claim "hey we were testing something new out, it didn't work" if you try and trust teams to show there full hand in practice. they might show there full hand in qualifying, but in endurance racing, qualifying and race pace are very different, and passing is much easier, so qualifying matters so much less than a sprint format.
to BoP naysayers, and i was one at one point, i said "let the companies and engineers battle it out without any BoP" ..... the reality is, the accountants win. whoever is willing to spend more money wins. thats not racing either, its spending money to win. GT racing needs BoP, simple as that.