About the lagging thing, there's an easy remedy. Open the default.ide file with notepad and after the cars there is a value like 'richfamily' or 'executive', this value tells the game when to spawn the car on the road. All you do is change this value to 'ignore' for the cars you have replaced, hey presto, they don't spawn on the road anymore and are now unique cars. Just make sure you have one in a garage before you apply this fiddle.
By the way, cars that always spawn in the same place are unaffected, e.g. if you change the model and texture on the INFERNUS but set it to ignore so that it does spawn on the road, it is still ALWAYS outside Diaz's Mansion, same with the CHEETAH near Cortez's boat, the COMET at the Golf Course, the OCEANIC outside the Washington Beach Hotel and the SENTINEL, DELUXO, SABRETUR, HOTRING and SANDKING at B J's Car Showroom.
Plenty of cars there to safely change without dropping the frame rate and although they won't appear on the road, you all know where to find them.
Hope this has helped.