I think it could be OK to trim like you have indicated. I don't think that it would make your beard wispy in this case, just narrower. That said, I think the way it is defined now looks good... so if it ain't broke...

Stop trimming your moustache; greater glory awaits!
...but if you are going to trim, do it right, go pencil...
...then, of course, a nice copstache can be cool too...

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