that exhaust leak might cause poor acceleration on a NA were back pressure is a good thing.. are we talking a PLUME of smoke here?.. but I don't see that being the issue. your dad is on the right track with the transmission. all small searches I did point twords your tranny going.. Do a tranny diagnostic.. what other codes are you throwing.
http://www.cygnusx1.net/Supra/Librar....aspx?S=AT&P=1
thats the link for the automatic transmission page for the Toyota Supra repair manual.. learn it, love it, live it..
check for codes on your tranny and report back..
other issues.
your afm could be the culprit..
check here also
http://www.cygnusx1.net/Supra/Librar....aspx?S=EM&P=5
theres a link for checking engine codes in the faq. or consult the tsrm.. its in there.
whats your idle sound like after you got back from the test trip.
if you have a normal idle of what is it.. 700 for an na?