The Custom is already lower than the Ultra, and at stock height I already scrape floorboards everywhere... I can't imagine lowering it. I know some guys who just simply 'cruise', and don't really ride their bikes hard, so I can understand for them, but I like to really ride, and push my bikes a little bit (within reason of course).
My 1st bike was a sportbike, so I learned with them. One of the things I learned was that messing with the suspension by lowering a bike is one of the worst things you can do. Engineers spend a LOT of time working out the geometry of the bike's suspension, and when you go messing with it, you can make the bike far less stable, and somewhat unpredictable when cornering. I won't take that chance.
I don't know that it is as pronounced on cruisers/baggers as it would be on sportbikes & standards, but even if there is a little wiggle room, there still can't be much.
I've found that the majority of folks who initially ask about lowering their bike do so because they can't flat-foot at a stop. You don't lower for this reason... you shave your seat. You can gain 1"-2" by having your seat shaved properly, and contouring the thigh area to allow your legs to drop sooner, giving you more leg room. And hell, while you're at it, you can re-foam or add a gel insert and make it more comfortable... all for less than the price of a Mustang or Corbin seat.
Lowering only the rear sounds like a good plan for some because they think "well, that won't lower the floorboards/highway pegs very much" but that's the least of your concerns. Scraping floorboards is no biggie, they fold and give... lowering the rear now brings your exhaust and bags closer to the ground, and scraping hard parts is flat out dangerous and can take your wheel out from under you mid-turn.
2012 Road Glide Custom Black Denim
Last edited by RGCBandit; 11-21-2012 at 10:12 AM.