Well, I'd like to update the forum about the findings. I successfully modded 2 exhausts, a 115 and a 135/160 one. I cut them open and removed baffles and tubing to make them similar to the Toyo (2 lateral chambers), although the Toyo has solid chambers separators instead of the OEM drilled ones. It took some careful angle grinder action and dremel finishing touches to remove everything while making a neat job. Then welded the lid back on and secured the air tightness with some high temp paste. Here is a picture of the 135 backbox just before putting the lid back on:
Now for the results. The 115 exhaust benefits from the mod in terms of backpressure, the car is more responsive, the exhaust pops a lot. The sound is better but not nearly as sporty or loud as any stainless steel backbox. This is probably due to the limited diameter of the tubes, the main one and the ones going to the tips. Throttle response on a 115 car is definitely better, especially at mid-high range.
The 135/160 exhaust mod went GREAT. Thanks to the larger diameters, the sound after the mod is almost the same as the Toyo. It does not drone at 3k rpm like the Toyo does (although I modded mine so it does way less). The perceived volume is only slightly lower but the sporty note is quite rewarding and it growls, pops and bangs. I LOVE the sound, less intrusive but still very sporty and definitely a head turner.
Despite removing a lot of metal (at least a couple of kgs) from the original unit, the Toyo is still way lighter than the modded OEM one.
However, I did this mod to be able to use the right tip valve, so in the next weeks (or months, time is always scarce here) I will create a circuit to control the valve for silent cruising.
It will be a simple knob, positioned on the "flap" over the ignition, with automatic, sport and comfort settings.
To do so, I will feed the valve controller with:
1) Nothing (valve always open, Sport)
2) Permanent 12v live (valve always closed, Comfort)
3) Original ECU feed to the valve actuator (valve closed at low rpms, open at high, Automatic)
Now I must find the right rotary switch, a low profile one with sufficient amperage/voltage tolerance. Any suggestions?