The tandem will feel a bit lighter when you pick it up, lighter than the actual net weight reduction. I'm not sure why that is, but it just seems to work that way when you drop 1.5 lbs to 2.0 lbs off a 30 lb - 40 lb tandem.

As for the ride, the tandem should feel more lively and, again, the perceived performance gain is greater than the actual change. This can come from a number of the different things that come with switching over to lightweight racing wheels and the extent of the change is tied directly to just how different the new wheels and tires are from the old wheels and tires, noting the tires often times do more to influence the change than the wheels alone.

Frankly, just going from your 48's to a very lightweight set of 36h conventional wheels with a 25mm tire would probably feel pretty dramatic, and almost on par with a Rolf as far as first impressions go: the smaller tire running higher psi + the lighter wheel set will not dampen road vibration like your current wheels and tires and when coupled with the lighter weight of the wheels = that more lively road feel. Where the Rolfs differ from even the 36h wheels is the spoke network: the radially-laces front wheel has very little vertical compliance as compared to the 3x 36h wheels, never mind your 3x or 4x 48h wheels which, again, contribute to the more lively feel. At the same time, having 1/2 as many spokes that are also bladed eliminates more than 1/2 of the air turbulence around the front wheel which you can sense by feel through your hands via quicker, lighter steering response and that you can hear, i.e., less wind noise.

These things all contribute to the perception that the wheels have made a profound impact when the actual net improvement is something a bit less dramatic and as likely to have come from the tire and tire pressure change as anything else.