There is some confusion about exhaust systems.
Your system is stock back to the "Y" pipe which is good, you probably didn't hurt mid-range power.
Any increase in power would have come from removing any blockages in the exhaust system, good mufflers and cats don't restrict or block exhaust.
Good Exhaust systems create a negative pressure at the exhaust valves, so aid in pulling out exhaust when valve opens.
They do this by using a pipe size that is small enough to get a good Velocity flowing in the pipe, but not small enough to restrict flow.
When this exhaust pulse hits the collector(larger pipe on exhaust manifold or header), it's velocity and pressure drops, this drop creates a low pressure at the other 2 pipes(V6) on the collector, so lowers the pressure at those other two exhaust valves, like a siphoning effect.
The size and length of the exhaust valve pipes dictates when the lowest pressure will occur in the RPM range.
So exhaust manifolds/headers can be made to give lowest pressure at low RPMs, mid-range, or high RPMs
There will be a specific RPM that has the lowest pressure, RPMs just below and just above this RPM will be the Power band for that tuned exhaust design.
Most after market headers are designed for lowest pressure at lower RPMs, that's what most people want, off the line power, stock manifolds are designed for mid-range power band.
Racing headers have the lowest pressure at higher RPMs.
This is also called a scavenging exhaust system, it is using free exhaust pressure to increase engine performance.
On dual exhaust systems they have been adding "H" or "X" pipes between the two exhaust pipes, increasing the scavenging a bit more.
The myth of "engines need back pressure" comes from this scavenging which has been used by automakers for 50+ years.
DIYers would put on larger header pipes and loose power, "WTF, free flowing exhaust and I LOST POWER!!!!"
"This engine must need back pressure"
The results they got were correct, larger pipe = losing power, but the conclusion wasn't correct.
They lost power because they INCREASED the pressure at the exhaust valves, they removed the velocity in the system so they lost the negative pressure at the exhaust valves the scavenging system had provided.
As said, in a well designed exhaust system Mufflers and Cats won't effect performance.
Bad mufflers or cats will effect performance.
Now sound is a whole other ballpark, lol.
No Cats or Mufflers does improve the sound quality, as long as you don't rev the engine when the baby is sleeping