Power draw's an apples/oranges thing because of the regenerative braking on the Sprinters. AEM-7's just radiate their braking energy off as waste heat, Sprinters generate their own electricity off it and put it back into the grid. What effect that has on the substations depends entirely on tolerances for spikes and dips in the load and how many trains total you're running on the substation. A regen braking vehicle's not necessarily going to help if it's more power-hungry on acceleration, and it's not necessarily going to help if it saves a little but the Silverliners are still stretching Reading to the limit. Given how few push-pull units SEPTA runs total, it's probably negligible overall. Silverliner VI procurement is still going to be the one that forces the issue with Reading.
As for car weights...couldn't find on Google any specs for the Comet I or Bombardier MLV. I did find these:
Amfleet: 110,000 lbs.
Comet V: 110,000 lbs.
Bombardier BLV (the original, non-East Coast double-decker Bombardier car): 110,000 lbs.
And as for seating capacity affecting that weight...well, no commuter consist in North America is going to be carrying an assortment of cafe cars, sleepers, and baggage cars like the NEC routes--Regional, LD's, or everything in-between--carry in-consist and which the Sprinters were designed to be overpowered to handle. On per-car weight Amtrak is still going to average heavier than a MLV packed with generally single-purpose commuters. Likewise, going from single to multi-level means needing fewer cars per consist if deployed on push-pull runs that are not perpetually sold out. If 5 MLV's have the seating capacity of 7 Comets, SEPTA's not going to match 1:1 car length on every pre-existing push-pull and just let everybody kick their feet up on the furniture. They'll shorten the consist if it wasn't previously overcrowded and apply the spares to an extra push-pull train (which would logically mean they'd benefit from 1-2 more locos padded to the order for added flex).
It's a spurious assumption that "MOAR POWER!!! = AWESOMER!!!". If you want the peak acceleration performance and Larry Leadfoot in the cab to floor it out of every station to save 5 seconds of schedule time you buy Silverliner VI's and don't even bother with another generation of push-pulls. SEPTA clearly has other obvious service efficiencies it gets out of running the push-pull fleet that are not acceleration-related, or else they wouldn't be buying them. I totally get it if it's simply cheaper to buy a software de-rated ACS-64 instead of a physically smaller engine for expediency's sake. But they're not passing anything up by going sub-6400 HP because of the very different hauling needs of an Amtrak vs. commuter train. And they would probably prefer a de-rated engine because it would rack up less overall wear-and-tear not pushing the components to their load limit on a commuter rail service with frequent starts/stops and a 90 MPH speed limit. SEPTA would have to midlife-rebuild these 1-2 times. Amtrak hedges on running 'em till they drop unless the units convincingly exceed their expected midlife condition (like the AEM-7's did) and merit the rebuild.