As mentioned, on the B division, the single unpowered truck per set is to facilitate the installation of CBTC related equipment. Specifically, an axle tachometer to measure distance travelled. This method is more effective than whatever the previous used was. That is why the axle the tach is installed on has to free wheel; no motor or brake shoe to induce possible wheelslip and throw off the distance measurement.
It's not going to feel different from a 100% motorized train because the motors installed are powerful enough to make up the difference in performance through a few software tweaks. The trains are scaled back from what they can really do anyway. We went from 4,600 hp (10 car SMEE) or 3,680 hp (8 car 75-foot) DC motor trains to ~6,000 hp AC motor NTTs on the B division, and yet the NTTs don't perform all that much better than the SMEE cars these days. I've heard stories of the OG NTTs (142s and 143s) hitting 70 mph on F5 track out on the flats when they were first testing, just to see what they could do, while today, everything I ride out there, SMEE or NTT maxes out around 47-49 mph.
On the A division, going to 70% motorized trains with higher HP motors dropped hp from 4,600 to 4,200, but the A division NTTs are a bit lighter than their SMEE counterparts (owing to the two-less motors on the B cars), so that probably accounts for the minute performance difference. The benefit is fewer motors to maintain, resulting in cheaper operation. The authority just took advantage of the presence of the trailer truck on one of the B cars to install the CBTC equipment onto when the time came.