Stevieturbo

A single GM coil was never designed to run at the sort of duties it would see firing an 8cyl engine, for all cylinders. It is a very bad choice of coil for a coil/dizzy application. Either use 8 of them, or none at all.

Often a little porting of the wastegate orifice makes a huge difference. Nothing major, just smooth the flow into it. And do not damage the seat the flapper sits against.

Scope coil current to determine what the coil wants in terms of dwell. triggering method likely inverted so what you think is the coil off time, is actually on time melting everything

Modern OEM Subaru COP with internal ignitors work fine even at only 30psi boost. 585's are over-rated, far too many seem to get a hard on over them despite their massive and dangerous flaw of premature firing with too much dwell. Your problem is a strange one. I've used IGN1A's on a few cars ( non-link ) and never had any issues with them. If you are not 100% sure the coil triggering system is working safely, I'd proceed with caution. Sparks firing at the wrong time, can cause big problems

either your solenoid/wiring or boost plumbing must be incorrect if it all worked before.

largely your tuner is full of it. The OEM ecu is extremely good, perfectly fast enough, and from the factory, extremely well tuned. His comparisons or analogies are not valid in the real world. In this instance the biggest factor as to your engine making power or not ( and doing it safely and reliably ), is the person tuning it. the second factor is the dyno and whatever numbers you actually believe. All dynos read different, all dynos can be made to read different, and dyno operators can use their dyno in different ways. Same with tuners and ecu's, there are good and bad, all too often bad is operator related. If he tuned your car on the dyno, and all external factors remained the same and you seen good gains from before and after, that's the primary function of using a dyno. Comparing to other dynos, other setups etc etc...can be a minefield.

LS2 is a silver blade, and on the OEM can be problematic over time, and are pretty much unavailable. They were only used for a short run LS3 used the gold blade, which is readily available.

A similar issue has popped up with other ecu's in the past. A sort of fix was to wire the supply for the ignition coils via the fuel pump relay or trigger. The slight delay in coils turning on with key on, prevented the initial fire some hear/feel. I highly doubt changing anything relating to fuel injected will make any difference, although the user feeling anything probably does relate to some degree about residual fuel already left in any cylinder. Which can often be down to tuning too.

If it's 1/4, you might get away with rethreading it at M14 for an ATS04 Or GM do a 3/8" NPT https://www.diyautotune.com/product/gm-open-element-iat-sensor-with-pigtail/

Would it not be easier to buy a sensor with a thread more suited to the hole already there ? What thread/hole are you actually starting with ?

Exactly where on the scroll is that ? The closer to the turbine flange the better. And how is the gate control plumbed ?

fit a normal camshaft and dump the vvt.

There is no fact that any setup can do that on every application, because every setup is different. But there is pretty much nothing a 4 port can do, that cannot be achieved with a 3 port. Nobody can guarantee you either will allow you that pressure range, and you will not know what it can do, until you try.

nothing wrong with the sound of the exhaust, I'm not hearing any misfires ? And I would not have used the quoted valve clearances...always go a little bigger.

Can't say I've heard any horror stories, and I know a few guys using 4 ports without issue. But really...a 3 port can do the same job anyway, been doing it on mine for years.