GTA-VC is one of those games that renders a lot of data on the fly. Your media and burn needs to be able to sustain a CONSTANT bit rate around 2X to ensure proper loading. For this you will need quality media at a solid 2X burn or higher. Test your copy's read speed in a straight DVD ROM (not a burner) with the CDSpeed utility mentioned in the Pioneer testing sticky. If it doesn't keep a steady stream up around 2X, you DEFINATELY should try either a different media or a different burn speed.
If it ain't broke don't try to fix it! But, hell if you can tweak that bastard just a little bit more....hehehe.