I installed a Celestion BN12-300S (4) driver replacing the stock 4 Ohm driver.
Observations so far in a bit over a day's worth of playing with it:
Even though it still seems like a fairly bright driver, the upper mids are not nearly as harsh sounding to my ears as the stock driver.
It is more efficient than the stock driver.
It has at least as good a low end (down to low 'B') as the stock driver, possibly better at higher volumes.
I have run my Cort B5 Element into the Instrument Input, set the combo's EQ fairly flat and then used my B5's onboard preamp to EQ a very good tone.
I have run the B5 in passive mode into my Two Notes Opus and then into the combo's FX Return and EQ'ed a killer tone using the Opus' processing.
I have run the B5 in passive mode into my Two Notes Opus and then into the combo's Instrument Input and EQ'ed a killer tone using the Opus' processing.
I greatly prefer the Opus generated tones into the R100c without any Cab Sim - which makes sense - less muddling up the tone.
I created 6 different presets so far - two for each of the above routing options (one Fender Showman type Preamp setting and one Fender Bassman type Preamp setting), none using anything than the Preamp types and the Custom 5-band parametric EQ + HPF & LPF (no Power Amp model).
[I also created two Presets on the Opus similar to the above but intended for a post EQ / post processing DI out signal, tweaked using my Sennheiser Headphones alone.]
All of that is to say, I highly recommend the Celestion BN12-300S (4) for anyone who has a 4 Ohm version of the newer R100 combos.
After all of this, in hindsight (20/20 right), I'd have saved up and bought a good quality active PA cab or something like the GRBass Active cabinets.
I still do really like the R100c, especially the fact that it is really easy for me to move around with my bad back and the Opus can make it sound like a thousand dollar rig (at least in that neighborhood)!