You would see a slight difference in a long and short barrel, but nothing that would prevent the gun functioning.
@400fps you are in stock USA gun values. All the guns we can buy at 350fps, well they sell just as well over the pond at 400fps. In fact, the guns we buy are typically downgraded to 350fps (TM Excluded).
Shorter/Longer barrel it makes very little difference provided it's volumed correctly to the gearbox. A longer barrel will have a slight FPS increase but it'll be marginal.
Heavier ammo is always a problem for AEG's. To hop it you need harder buckings so they don't wear out as fast, but also more accurate hop control so you are not constantly over/under hopping. It's why hops that can be locked in position are king in bolt action guns. Do you need that @400fps, nope. Simply moving from soft 50-degree rubber to a slightly harder 60 degree should be more than enough. If you find the 60 wears quickly then you simply go up in hardness again to a 70.
You are not going to be able to hop sniper weight BB's in an AEG based DMR. 0.43g BB's might have the weight but they certainly don't have the speed at 290fps. Add in distance and you will have many problems trying to hit a target. A more realistic value would be 0.3 - 0.35g BB's that will keep the gun over 310fps.
A bolt action shooting at 500fps with 0.2's (2.37J ) is still getting 350fps with 0.4g bb's. It's the combination of the speed and weight that work.
The best modification you can make outside of the hop rubber, inner barrel and mainspring would be a Pre-cocking mosfet. It'll take the gun and make it instant.
I personally don't feel there is any reason to go beyond this, and upgrades to 400fps DMR specs for the UK market are pretty much just clever marketing to make you part with cash for no reason.
