My 2c is that usually you would pick whatever ecu your tuner/workshop is comfortable with using so that you have a 'one stop shop' for future upgrades/tunes.

However cheapest option, given that you already have a ecutek tune, is to bring it to one of the workshops mentioned above.

So how does the Possum work with no AFM, Does it come with a Map Sensor?

As I said in the PM, it's got a little black box that comes with it and it's fully tunable as far as I know, some like them, some don't but when they were new they cost as much as ecutec if not more, Mao's tuner would be best to talk to if he knows them.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.

Apart from not being able to retune the Ecutek with another program you wouldn't know it's on there, only highly experienced subaru dealerships can tell it's on there as it stops none of there equipment working apart from updating it with a new tune.
As for your problem, sounds like a an up-pipe.
I can guarantee it is not the tune, no workshop would let it leave running like that unless there was a mechanical issue the customer did not want to fix/pay for.