You will definitely get a better sound with NOS Mullard tubes..........more euphonic, more complex, mid-emphasis.
I don't know how to respond to your "10-20%" improvement.
Nothing gives you increments like that, unless you are starting with pretty poor gear.
To me, 5% is fantastic......the difference in good and great sometimes on a vocal track.
It is more the character of the sound, which is definitely better with NOS Mullards.
I didn't even consider using it w/o NOS tubes.........the same for every other high end tube unit I've owned, for that matter, except for maybe the Massive Passive.
They will give you a very noticeable improvement, IMHO.
If you spend enough to buy a Reissue LA2A in good condition, then pay the extra to buy the NOS tubes.
Go to Upscale Audio and buy them from Kevin Deal:
Upscale Audio - Rare Tubes
You will always get good tubes here........he does a great job of testing.