The trade off is more THD at the analog stage and more dynamic range to tape vs. less THD and less dynamic range to tape. Given that most pro A/D converters yield a usable 110+ db of range, I say don't push your analog gear so hard
The long answer is that most vintage / vintage re-issue gear was going to tape, often calibrated to 0VU = +4dbu - now people usually used 0VU as a starting point, and pushed a little harder, but +18VU was serious clipping even on a studer 827 - which (given the previous calivration) would be equivalent to +22dbu. So I'd say that calibrating your meter to +18dbu = 0dbFS is a reasonable medium. Calibrating to +22 or +24 becomes more reasonable to me if you're someone who shoots for -12 or -18 when tracking.
So, nothing is wrong if you continue at +15, unless you then also shoot for like -18 in that environment at which point you're loosing a slight to modest amount of dynamic range, which (in 24 bit with decent converters) also ISN'T REALLY A BIG DEAL for most types of music being recorded today.
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