How much velocity I can get out of this cartridge in reality? I know its brother, the 3 inch version is loaded mildly aprox to 2100 fps. Is it possible to get ca. 100 - 200 fps more taking into account the voluminous case? I have the feeling the Ruger No.1 could handle it...

Yes, no problem getting another 100-200 fps in the Ruger No.1 for a 450/400 NE 3-1/4", but not a good idea IMHO.
The rifle can certainly handle it.
The old original brass is certainly weaker than the old original 450/400 NE 3-Inch, aka .400 S. Jeffery.
That was the greatest thing about the .400 S. Jeffery:
Designed from scratch with thicker rim and head, and thicker brass all around, for the new-fangled Smokeless powder in 1898.
Modern-make brass?
I have not sectioned the 3-1/4" case, but it still has a thinner rim, has to, directly descended from the old BP and BPE cartridges, fits the old rifles.
I know the Hornady 3"-case is good for 60,000 psi.

If you don't already have the 3-1/4" version in your Ruger No.1, don't do it.
The 3-Inch version, .400 S. Jeffery, is a much better idea.
You can easily load .410/400-grainers to 2400 fps in that.
You can also load .410/210-grain Hornady XTPs to +3200 fps.
And you can do all that with less powder in the 3-incher.
I do not know if the 3-1/4"-incher brass could handle the pressure if you added enough powder to do that.

Rip ...

(IIRC) Brittany Boddington's Ruger No 1 450-400 for her hunt and articles-
was throated to seat the bullet out further and to accept more powder-

That is the simple solution given the 3.25" brass availability and strength issues

Heck, there is no solution needing to find a problem here.

Brittany used a .405 WCF Ruger No.1 to get a 400-grainer going 2150 fps to make it legal for a successful cape buffalo hunt.
See May 2008 G&A article about her overcoming recoil issues when learning to shoot with her Pop.
She was not looking for more than that.
The article mentions only the strength of the Ruger No.1,
though re-throating of that one might have been done also.
IIRC, her Pop might have used it too, might have developed it for her, along with his .400 S. Jeffery 3-inch Ruger No.1.

Rip ...

you are correct

my memory was blending two articles-
it was the 405 that was throated, not the 450-400

I should have remembered that,
having previously throated my 1895 Winchester(Browning/Japan) to more easily accommodated 400 grain loads

quote:

I should have remembered that, having previously throated my 1895 Winchester(Browning/Japan) to more easily accommodated 400 grain loads

Me too on my 1895 .405, so the 400 grain .411 Woodleigh would crimp at the cannelure. Loaded to 2076 fps MV, it handily took a Cape Buff in RSA.
My Simson Suhl .405 double is regulated for Hornady 300 grain bullets, but it also shoots the 400 grain Hornady and Woodleigh bullets well at 2050 fps.

I had both at one time or another.many times over, the best I could ever do was 50 FPS, and most only got me 25 FPS. That's the reason to own the 3" and you will save money on the cost of brass and have more to chose from in available guns..My experience is totally with double rifles so in a Ruger no. 1 I suspect I would work on no more than 100 FPS if for no other reason than to save brass and get more loadings, its pretty soft stuff to start with in the 3-1/4 version or at least thin, less so in the 3" case

Both are excellent calibers and if you handload there is no difference in the two, other than the availability of good brass, and the cost of brass all in favor of the 3" chamber.

I would also consider the 450-400 in either case will serve you no particular purpose by going for more velocity, and probably give you less penetration as the bullets are mostly designed for 2100 FPS, long before the advent of the Ruger no.1, and that's has not changed, its a caliber that has always been built around double rifles and a few old single shots, and never has it been updated..Never has needed improvement,ya can't fix perfection

Then I may be off track, you may intend to just shoot it and not hunt DG with it..in which case give it your best shot and let us know how much velocity you gained in the big case and strong action.

With buying an older double rifle chambered for the 3-1/4 450-400 one must make sure it was originally proofed for smokeless loads. The 3 1/4 450-400 was offered in black powder only, nitro for black, and full nitro. It should have the proper proof marks. the 450-400 3 in was never offered in black or nitro for black.

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71 grains of RL-15 gives 2180 fps. However I never needed more than 2100 I'm getting with 69 grains.