For the gains it is not worth the work, building what you can buy never makes sense.
Not strictly true. If you have a shell that has had work done to it and you know the condition of it inside and out then it's worth doing an engine conversion rather than buying a car with that same engine but having to re-do every modification on the chassis side of things.
But unless the shell has had major performance upgrades done to it, or unless the shell is *literally* rustproof then it's not worth, as converting a non GTI model to GTI spec is a lot more effort than converting say an 8v gti to 16v or VR6. You'd need engine, pedal box, gearbox, shift mechanism, probably fuel pump (not sure if GTI and non GTI is different, but I presume it is, unless you non GTI has MPI). That's just engine side, you'd need to upgrade brakes and suspension to cope with the weight gain.
As I said, unless your shell is
literally rust proof or it has had a lot of work done to it, then it's not worth it.