So thought I would deliver a bit of an update. Took the Spectral out for a spin on the trails after some rain (naughty naughty) about 1 1/2 hours worth of unfit, heavy breathing, slow peddaling muddy nonsense (had a blast, sorry not sorry).
No pack and loved it, sweaty but not super sweaty had just enough water in my 700ml camelback water bottle for that ride and my BackCountry Research performed flawlessly and silently. The tubetarp kept my tube clean and the racestrap kept my tyre levers, 2 CO2 inflators and the tubetarp wrapped spare tube well secured and completely immobile.
Highly recommend both those products. I think with my new (on the way) High Above Cascadia Hip pack (with bottle rocket) and another 700ml water bottle I doubt the pack will come out for anything under 3 hours.
Tell you what I now know why all those Endurobro guys ride with no pack, it feels so free like going commando!