exhaust mod, need some advice

[hex999]

Recently decat my cefiro a33 2.0, used back original diameter which is 2inch, 1 silencer in the middle and 1 mufler(1in 2 out) at the back, and 2 exhaust tips behind. After decat, FC improved alot, low end torque improved(just need to tap the accelerator a bit to move the car compared to last time), high end torque almost the same. Im quite satisfied with the setup only the thing that sucks is, when i release the exhaust, the RPM drops instantly and the 1 in 2 out exhaust which has 2 slow characteristics sound loud at low RPM, quiet at HIGH rpm, which I prefer it the other way round..
The piping right below my manifold has turned purple which is what im worried too because i believe its too hot, or is it normal?
I was wondering what should I do to improve my current setup? Do i need more backpressure or less?
I previously wanted to use 2 straight flow muflers for the rear end but i was afraid i would lose all the lower end torque. need advice from any exhaust sifu :D

 

[hex999]

Updated, tested with OD off, the RPM holds itselfs and goes down gradually as usual after letting go accelerator, but on normal drive, after letting go of the accelerator.. it quickly drops instantly.. for example 3k rpm to 1.5k rpm in a few seconds when cruising at 120kmh... engine rpm is not synchronized properly with the gearbox i guess only when foot does not touch accelerator..
also checked for diagnostics and no sensor errors or fault code detected