I haven't owned a supra in 5 years but I see the flame wars I used to see on the sogi mailing list haven't stopped.

Anyway, if the 1j is a map car it might run similar to the gen3 mr2 engine. When I was running the stock downpipe I would be more prone to boost cut because of boost spiking. I think the backpressure caused some ping, set off the knock sensor which retarted timing, and led to boost spikes. I wouldn't be surprised if a better open downpipe might help avoid boost cut.

As a side question, can you put stock 2J turbos on a 1J? Might be pointless, but I'm just wondering. thanks!