Where back presusre becomes an issue for overall power is after the turbo(s), not between the engine and the turbo, once the cars at target boost and the wastegate is fully open theres almost no back pressure here sense the pressurized exhuast coming out of the engine is just going right out the wastegate. This is the number one reason why boost drops off a little bit in the upper rpms since there is alot less back pressure now so it reads less but you still make power, this is also why dumping the wastegate to amtosphere is more efficient than routing it back to the downpipe and you can pick up a little power this way.
My setup made 604rwhp at 14psi with a.96 ar housing. I went to a .68 housing and still made 605rwhp at 14psi and increased spool around 200-300 rpms. And my entire power/tq curve was higher through out the rpm range. This was with no changes to the wg spring or boost controller. At relatively low boost levels ie 10-20psi turbine flow is more importnat for efficiencey and spool. Its when you start running 20-30+psi where a turbine with not enough flow can hurt peak power. On a v8 at the 15psi theres really no power loss. You can get away with the larger exhuast housings since you have 2 turbos so spool isnt hurt as much but those larger housings arent gaining you any peak power over say a .81 or .68 housing.
Boost at the manifold is just a measure of restriction and is almost completely centerd around how well the engine can take in and push out air. So yes the manifold, heads cams, engine dispalcement and exhuast does come into play. But the biggest factor here is the engine. I think you are a little confused and seems like your trying to say on someone else setup it would read more boost to explain the power, but this its just not the case. It would more rely on engine mods like heads/cams work, compression, stroke etc.
Your never going to have a 1:1 ratio unless you have a 6" diameter downpipe dumped to atmosphere right after the turbo. Besides 1:2 is ideal bp, your turbine does need some back pressure.
Hows your boost curve look? It will tell you alot about how the setup is working if you have dyno graphs to post up, also what are the specs on the motor?
Edit i see on your vid i says stage 1 cams so thats the reason you only see 14ish boost and make more power. But your setup has way too big of piping and no back pressure and spools so ridiculous slow. You dont even hit full boost till jsut before you let out. A twin turbo setup should make full boost way down low in the rpm range. I have an eboost2 as well love it!
YouTube - Twin turbo mustang 14.5psi‏
Here is a vid of my single setup spooling.
YouTube - 07 Mustang 67mm billet turbo spool‏
YouTube - 07 Mustang GT Precision Billet 67mm turbo‏