Are these the original injectors? How many miles are on them? Injectors can suffer from several different problems. Carbon deposits on the nozzles, mechanical wear on the internal parts, clogged fuel inlet screens, wear on the pintle valves, bad solenoid windings, etc.. You might try doing a leakdown test on the injectors first to see if any of the pintle valves are leaking or otherwise malfunctioning. If the injectors fail the leakdown test, try using a good commercial solvent in the gas tank (such as Stoddard solvent) and see if that helps clear out the nozzles and free up the pintle valves. Chevron sells Stoddard solvent under the trade name "Techron" and is available at Chevron gas stations, most auto parts stores, and places like Wal-Mart, K-Mart, etc.. Try using one container of solvent in your gas tank on your next fill-up at the gas station and see if you notice any performance improvement as your tank nears empty.
I have a photo documentary on my web page of when I pulled my injectors to replace the O-rings, gromets and insulators. You will need to pull the intake plenum (including all vacuum lines in the area) and the distributor. If you pull your injectors you should replace all the gaskets, crush washers, O-rings, etc as they are cheap and you are in there anyway. You can still get factory injectors for the MkII from toyota (in fact I got all my parts from toyota), but I paid about $130 U.S. for the 1 injector that I replaced. The job isn't hard, but you need to go slow and label everything to make sure it goes back together right. (At least I needed to do that anyway )
Just click on the (WWW) on my post and you should get to my site. From there, click on the Supra page, and then the "Fuel leak Project" page. If you have any questions about the disassembly I will be happy to answer them to the best of my ability.
you can take your injectors to a fuel injection shop and have them sonic cleaned as well. They'll do a flow test before and after and rate the spray patterens. Injectors can get gummed up pretty bad with age. It only takes one injector that flows badly to cause bad gas milage. If one or two injectors are leaning out a cylinder or two, then the o2 sensor catches it and enrichs all of them. After changing every common wear part that can cause bad gas milage in my wifes ae86, and still not getting better then 19mpg on the highway, I rebuilt the intake manifold and had the injectors cleaned at a shop. It cost 25 bucks each injector (thats with new filters) but when all was said and done we were able to get 34+ mpg. 2 of the 4 injectors were flowing about 25 percent less then factory spec.