45-70 vs 45-90 with moderate smokeless loads?

[kombi1976]

I have a Martini Enfield MkII action which I'm intended to have chambered to 45-90.
As many would know the Martini Enfield is best limited to 45,000psi as this is the max pressure developed by the military 303 174gn FMJ MkVII load.
I originally considered having it chambered to 45-70 but as Starline does 45-90 brass suitable for the warmer smokeless loads I thought the extra room would help.
Has anyone had experience loading smokeless in 45-90?
What sort of velocity increase and therefore improvement in power will the extra case size yeild?
BTW, for those wondering why I have not chosen to chamber this action to 577/450 the reasons are 2 fold.
Firstly, I'm unaware of any gunsmith who has a reamer for '450.
And secondly, and more importantly is the cost of brass.
Bertram here in Oz makes 577/450 which costs $128 for 20 CASES!!
While the old '450 may well be able to equal and better the 45-70 with modern smokeless loads the running costs are staggering so I'm giving it a miss.

[Smokin Joe]

I replied to your post in the other forum; I shoot a '90 with smokeless. I re-chambered a .45-70 Buffalo Classic just to get more powder room, and it does work. More gas at the same pressure equals more velocity.
 I can push a jacketed 400 gn past 1850, but it's not fun. I usually shoot cast lead at about 1400.