for $130 for the whole set.. I have installed a lot of them and they fit perfect. The downpipes can be removed with the engine in the car but I understand your mechanic might not be very experience with the 300zx. You can ask him to drop the trans to make the downpipe removal easier.
Also, did he install them correctly? This might sound silly but you can double check.
1990 Nissan 300ZXTT Twin Greddy TD06L2-20G 745rwhp 593rwtq no tuning (26psi no nitrous) 7.29@108mph 1/8 mile 10.9@139mph 1/4 mile (1.75 60ft) Waiting for stronger wastegates spring