IMO, removing the lowers will have little impact on the cooling of the engine when it needs it most...sitting in traffic...and no impact when moving along at a reasonable speed. Now the effect on the rider comfort may be a different story, when moving. There is plenty of air coming through the vented lowers when moving down the road...to cool the engine as much as it can be cooled given the ambient temperature. No doubt that the lowers block some air from hitting the driver's legs.
Go sit under the hood of your car one day and see how cool it is. We're all sitting with our most tender body parts inches away from a good sized heater, not to mention the exhaust temps. It's hot. Even a water cooled Vrod, though the exhaust doesn't have the same effect because of loaction, is hot in the summer...if you don't believe me, get on one and have the radiator fan come on and blow that hot air back and up in your face when it's 95 degrees and 80% humugity, like it gets here in Birmingham, AL. It's hard to breathe, the air's so hot.