Thanks! This is quite interesting.
Last Sunday I fitted the modded Toyosports and found that after the modifications (less backpressure and more free-flowing design), the resonance at 3k almost disappeared, making it more comfortable to drive at cruising speeds. So I might keep it in the end, but still unsure about it. It is VERY addictive now. VERY.
My main concern is how the sound of a standard backbox can become by altering the internal structure, don't want to ruin a good (and rare!) valved-backbox if it's going to sound like a fart
I might consider sourcing a more common 115 exhaust and, although with smaller pipes, see how it turns out before altering the 1.8 one.