I disagree that no one will notice it. When we did it last April, it was VERY clear that there was "something" going on. I would estimate about 20% of the park's populous were dressed for the occasion. It was noticeable enough that someone on our bus back to the resort that evening chatted us up regarding how we were aware of the event, etc.
But...
The vast, vast majority of those participating are AP holders, locals. If you're familiar with how the locals tour at WDW... they're more likely to wander, to visit with friends, to catch a spin on Haunted Mansion, Pirates or the Peoplemover than to book a FP for 7DMT 60 days out. And they're sure not waiting in that line! In fact, about the only line they'll wait in IS the Peoplemover... (I attached a photo from last year. Yes, that's an extended queue rope.)
Check the event website shortly before you go. That time, there were windows - carousel 12-1, Peoplemover 1:15-1:45, etc - in which the groups "met up." The event has simply grown too large for a true meet up, and it grows every year. The Peoplemover photos are during the designated windows.
The net effect of this is that although you may see higher *crowds* you will probably not see longer *lines.*
It's a fun atmosphere, so I wouldn't let Dapper Day dissuade you.
I hope that helps.