Injector/coilpack isn't going to fix the compression. Compression checks are the same regardless of engine layout: pull fuel pump fuse, plugs out, throttle open, compression tester hooked up to cylinder you want, crank it over.
The shop should have put a little 30wt oil in the cylinder and redone the compression check. If it improved slightly it's your rings, otherwise it's a valve. If both cylinders on the same bank are low it could be a head gasket.