On the SR300EB I had, it would become unplayable below D Standard.
OTOH I had a SR300E (no B) that had a very stiff neck, but the local luthier pointed out how twisted it was.
It's kind of the luck of the draw, but I think more SR models are going to have flimsy necks than the BTB series.
The odds of getting a nice stiff neck on a sub-Premium SR are low these days, but that's just my assessment from a couple. (and also hearsay)
Once you get into the Premium & Prestige line, they've got Titanium reinforcement bars & are stiffer & more stable.
You can get a pre-owned Premium SR for under $1K USD.
I'd recommend the SR1400TE. (neck-through)
The 300EB comes with a 5 piece maple/rosewood neck. The 300E, 5 piece maple/walnut.
I'm guessing here. But I would say there's your difference in stiffness between the two necks.
The neck on my new 2018 SR400EQM (purple space burst) is really stiff, fast, and exceptionally stable, especially for Alabama and all the high humidity we have to deal with. And it's been getting dragged out of the house to the car, to the bar, and back, in a gig bag, in sweltering humidity, rain, and now the cold, every week since the end of August. I also have no problems tuning down to C and keeping it there. I haven't tried dropping to B, have had no need too. It's neck is 5 piece, maple/walnut with a Jatoba fretboard.
Now there is a SR400EQM (Dragon Eye burst), kind of a tobacco sunburst, and it is the same guitar as mine except for color, and neck. The neck on it is 5 piece maple/rosewood like the 300EB. It would be interesting to be able to sit down with both versions of the SR400EQM side by side and see that the playability differences are. And there is one more version of the SR400EQM that has the same maple/walnut neck as mine, but a maple fingerboard instead of Jatoba.
And here's the really interesting thing about the SR300s vs the SR400s. They are exactly the same, come in the same wood options, electronics, etc... except for... the 400s have quilted maple tops. Other than that, they appear to be the same guitars.
Maybe it really is just luck of the draw when buying guitars these days. The majority are mass produced all over the planet.