I'm pretty good at tinting and still I only DIY on my work PU trucks and vans. Any half-way nice car I take to the tint shop. I have an excellent tint guy in the SF valley that does it while you wait for a fair price and the work is flawless. Best ~$140 I ever spend on a car. I cannot live w/o high quality tint on a vehicle now that I'm used to it, feels like I'm a friggin' ant under a magnifying glass w/o it in L.A.
What the others said about the targa rear window; it's one of the most challenging car windows to tint ever made. Drives the pros batty. You would have zero chance of doing a good job. On any Porsche you want a flawless job, so take it to a good shop. There is nothing that looks worse than a bad tint job, and few things look better than a flawless one on a nice car, IMO.
(FWIW, the only vehicle that I would not tint would be something very collectible that I wanted 100% original, like maybe an early 911S. At any rate, it's a reversible mod).