You could have got it to +5, then use the +10 Coupon. Just sayin'...
There's no reason to do that as the coupon makes the item +10 regardless of it's a +0 or +5 or +8.
Even the temporary power boost wouldn't matter, as they wouldn't be able to use it due to not meeting the Lv. 70 or r6 Mastery requirements.
Anyways I used mine on a random Twin Saber I bought. Got to +11, then it blew up. Wasn't really worth stopping at 11 for me. Sort of wanted to try to +11 an Infinity, but there's no reason to waste money on that now when prices will inevitably fall. That and I'm not that rich.