GP == Grand-Pa
In general as you go to a larger rim, you go to a smaller tire, thus the circumfrence of the tire/wheel is the same. (plus sizing) Your speedo is usually calibrated on this original sizing also.
The problem with custom wheels, well cheap custom wheels, is that they are cast wheels. As such then are cheap in design, and usually made of the cheapest aluminums. Thus they dent easily and can be damaged out of round with heavy potholes. With less rubber to absorb an impact, well you are certainly more likely to damage one.
These rims, 1 piece forged Evo's are basically a piece of billet aluminum that are cut (cc'd) afterwards. There strength and integrity are some of the best. I believe this wheel was originally a BC brand wheel, and they were purchased by Weld and renamed the Evo line.
Now with a three piece wheel (like my vettes) it actually combines both. It has a replacable/repairable barrel and forged center for strength and they are usually considerably lighter. Originally designed for racing applications, they have become much cheaper and are well regarded in custom applications.