Johnboy i beg to differ. i have 4*6 speakers fitted at the moment [they where spare sitting around in the garage] and i had to make up some adapters as they are way too small for the existing mountings. admittedly some 5*7's are oversized so may not appear to be a direct fit and some 4*6 have aditional holes for a 5*7 fitting. i believe the infinty 5*7's also fit a 6*8 opening for example.
taken from lukes instal guide in the link
Replacing standard size speakers
The GTV has essentially six speakers. Two 0.75” tweeters and two 5”x7” oval speakers in the front doors and two 6.5” round rear speakers. All of these can be directly replaced, although you will need to remove the door panels to change the tweeters in the doors.
Removal and refitting of the main speakers is quite simple. Regardless if you are working on the rear or front speakers, the process is essentially the same. You need to remove the grille first. Do this by inserting a large flat-head screwdriver between the grille and the interior panel at the side, then push the grille out, move around the grille doing the same at the bottom and the other side, the grille should just pop out at this stage, without having to use a screwdriver for the top grille-clip.
Once the grilles are off you’ll see four screws (front speakers) or four nuts (rear speakers). Undo these, disconnect the cables (taking note of which colour is connected to the positive and negative terminals) and directly replace them with your new speakers using the same screws/nuts. Once the new speakers are in place, refit the grilles and you’re done.
i assume the Cup is the same.