That is a rare build, really. Rather apex GT-Four what with the Caldina sourced 4th gen 3S-GTE. I bet it rips, and I bet you are jazzed af. I hope it works well for you!
You didn't really say how it's set up, so I'll share how my ST185 is set up. The example I just bought came with an ancient Blitz Twin Boost Controller, an Apexi 5-channel SAFC, and an Omoni Boost Gauge with a sun shield so old it was crumbling. Which telling inspires me, tonight is the night I just go out there and tear the rest of it off the gauge. I have some kind of aftermarket exhaust. I'm going to look around under it and see if I can find what manufacturer--it looks like the HKS Silent Hi-Power. This equipment provides the capability to achieve 1 BAR / 15psi, but I am still re-breaking all the seals back in after who knows how long of sitting unused, so the boost controller sits unpowered and I run stock .6 BAR boost at max for now. It has some other stuff--a set of Cusco pillowball upper plates, and a Cusco rear tower bar to stiffen up the back end. I still need to really look into the suspension to see what the deal is with it--it seems a bit lower than I remember my long lost 1991 Celica ST.
I think if you have a clean, unmolested example, you could unlock boost--and better--by the link ECU you mention and a tune session. Everything's possible with money, right?https://www.gb-ent.com/blogs/news/ecu-v ... ontrollers
After you get it as mechanically sound as you can. I don't want to disparage the seller or your pre-purchase inspection, or be a downer, but things happen. I had all kind of ideas for mine--Yellow Racing Gravel Coilovers, a Front Mount Intercooler, a larger downpipe, maybe an ATC Streetlite flywheel, maybe a carbon fiber single piece driveshaft. But my improvement money was eated up by rather mundane stabilization costs--fluid changes, timing belt kit, water pump, new alternator, new tires, new battery, converting and recharging the empty ac system, detailing tools and supplies, yet to be installed temp and air-fuel gauges...you get the picture. Definitely maybe eventually!