The H-Pipe really plays very little in terms of performance- nor does it reduce backpressure in any way- it was originally introduced to equalize the pressure between the two pipes, inducing a scavenging effect, but in reality, while it did improve some things a little, all it really, the only thing it really was effective at was to reduce noise- nothing more.
An X-pipe will increase HP if it is installed at the "area of confusion"- but that area is identified by painting a small strip on the exhaust pipes about 1/2 way between the engine and the axle- the area of confusion will cause increased heat and blister/burn the paint at that location- that's where the H or X pipe would be installed- and eliminates the air flow disturbance by sending alternating pulses in that location which acts as a "vacuum" if you will to push & scavenge the remaining and incoming air out.
In regards to the "High Flow Cats"...... don't buy into the PR....there is no such thing as "High Flow cats"!
Here's a link to a "Real Test" that verifies....on a 700+ hp engine......no gain or loss of with or without the cats!
The myth about high flow cats
Catalytic Converter Research News - Clearing Up the Myth About High Flow Catalytic Converters
As far as deleting the downstream sensors...... a very, very bad idea even from just an ecm management aspect! Production calibrations have a “protection model”
, based on many PCM parameters
, such as engine speed, load, ingested air mass, time, inlet air temperature, EGR flow rate and many others. The PCM will richen the A/F mixture as necessary to lower the exhaust gas temperature but will also eventually make what a software engineer would call, "An erroneous conclusion"... in short, causing "Exhaust valve tuliping", ie catestrophic engine damage.