Hi Colin,
I always use a uniflow tank, but not always muffler pressure, I try both ways to see what works best. a Standard Vent tank the fuel pressure and flow will change making it leaner and leaner as you fly.
In a profile model, I like to use Sullivan 4.0 OZ tank plumbed for uniflow, but I guess a 4oz tank will not fit in your model and a 2oz tank may be too small.
I suggest you use a metal uniflow tank that is the narrowest, so the fuel pick-up point is as close as possible to the NVA.
If it is still sagging on top, perhaps you need a bit more power. You can change one thing at a time like more nitro, a larger venturi, or a less restrictive muffler.
Sagging could be related to fuel delivery, doubt it, but it is worth mentioning.
I never used this Brodak tank, but I understand it is the narrowest one you can get and it was designed specifically for profile models.
https://brodak.com/fuel/fuel-tank/brodak-tanks/uniflow-large-profile.htmlFor a profile model, you always start with the tank 1/8" higher than the center of the venturi, or better yet of the spray hole, that sometimes is not centered with the venturi. And then make adjustments if necessary after test flying.
Good luck,
Martin