As a simple lay person with no sub or defense experience I can offer this discussion.
Tesla can offer mind blowing acceleration and huge range in a vehicle of regular dimensions because of Lithium batteries. They will be lighter, smaller, handle regular deep discharging, supply more current and have significantly more capacity while doing it, charge quicker, than conventional lead acid batteries.
The pumpjet is not as well suited to lead acid battery Diesel electric submarines, because a short sprint will exhaust their charge very quickly, in 10's of km. Lead acid batteries don't actually deliver their best when they are providing huge currents, you actually get significantly less total energy. Many batteries are rated in capacity (Amp hours) and this changes depending how much current you pull out (significantly different at high currents).
Lithium batteries can provide high current with less loss. So Lithium batteries in a car application can deliver a large amount of current and do so more than 3 times more effectively than lead acid batteries could in these high demanding applications.
http://corporate.siemens.com.au/con.../apdr-october-2015-issue-future-submarine.pdf
All three CEP contenders are likely to offer Li-Ion
batteries as an energy cache. Li-Ion batteries have significant advantages over standard submarine lead acid batteries. They have a higher energy-density and much lighter weight; for slow speed operations they offer approximately 1.3 times capacity, but for higher speeds they offer up to three times. It also makes them ideal for pairing with high-powered diesel-generators during rapid transits.
Also more efficient high-speed runs can be conducted deep with regular snorting periods. Li-Ion batteries are also able to take very large charging currents over the
entire charge range of the battery compared to lead acids, which can only be charged at full rates up to about 85% after which time the charging rate must be reduced to avoid a dangerous gassing situation.
The SB design as it stands now can't really make good use of its pumpjet, nor the most of its 4 diesel engines with conventional lead acid batteries (IMO). However, lithium batteries are coming to subs. Incorporating enough lithium ion batteries to be able to take on propulsion loads would really open up this design (IMO). The french are probably the furthest behind on lithium batteries, as its not critical when you have SSN, and for most SSK's fuel cells will do the job for their CONOPs. Japan with its dated Stirling AIP engines is right on the cusp of deploying lithium ion batteries en mass on sub fleets, so they would be the ones to talk to.
While the SB is much larger than the upholder, I think it would be an interesting exercise to look at hydrodynamic drag compared to say Upholder or Collins. Short fin Barracuda is only 1.6m wider (8.8m) than Upholder, and the drag is probably not a big increase as you may have initially thought (not that frontal area is the only concideration and not that reducing it to a simple cylinder is accurate). With retractable planes, pumpjet and improvements in design and construction I would imagine it would be in the same order of magnitude. I assume the french were pursuing these in trying to reduce noise and also perhaps to extend the time between reactor refits by reducing drag (?).
Its an interesting choice, there is potential, much we don't know about and it seems there may be more that will become clearer in the future. It seems like the RAN really did want a conventional SSN, and the end result of this may be a very different type of sub from what anyone has really built before.