I ride our tandem with my much lower wattage wife. From our climbing rate in the mountains I was able to calculate our combined wattage and thus my share of the work, because I know my climbing rate on my single on those same roads. For me, it's the same as riding a very aerodynamic single bike weighing 114 lbs. We ride our tandem a lot, in fact I almost never ride my singles any more. So this illustrates your question very well.
The answer is that it changes your strengths, but doesn't make you faster. On my singles, I'm about the same speed on flat and climbs as I was before we started doing all this tandeming. However now I have a tendency to spend a lot more time out of the saddle on my single, especially on shorter climbs. In the gym, I use about the same weights for squats and leg sled that I did before, so my legs are not technically any stronger. However they do respond better to short efforts requiring strength. This difference is so small that I don't think you'd get much out of a 40 lb. bike. But riding a fully loaded tourer in the mountains at lactate threshold might cause some adaptations. Whether those adaptations would help or hinder you would depend on what you are trying to do and your personal physiology.
Gearing on a heavy bike is very important for creating adaptations that might transfer to a lighter bike, because cadence must remain the same as it is on your light bike. We run a 26 X 34 granny on the tandem. For example, training on a single speed bike does not increase one's speed on a geared bike more than doing the same training on a geared bike.