First of all, you're comparing two very different cars. No one, and I mean no one, who cares about the handling characteristics of a car will opt for a GSF. All Lexus F cars (other than the LFA) are heavy, bloated and badly tuned for handling.
The MX5 is miles beyond the GSF in the driving fun department and is perfect for the mountains. I would never own a GSF but I'll definitely consider a MX5.
I have a few questions for you though. 1. I recommend you getting a Chinese capable keyboard because your English is not that good and this is a Taiwanese forum. Plus I don't buy the whole I can't type Chinese bit. 我現在就是在用沒有注音的鍵盤在打中文 2. Why are you treating this forum as your personal Facebook status update? Can't you put everything in one post? 3. A lot of the things you're saying doesn't even make sense. Like no one asked if your wife is a "tasteful" person.
Today I just drove my GSF on mountain roads again to tell the differences when I still remember that Miata handling...
Wow! I felt my GSF handling was actually very good... On mountain roads my GSF felt so nimble when comparing with that Miata... I was really surprised how come such a big sedan could be felt so nimble...
When I changed from M4 to my GSF I didn't think GSF was that good... I think maybe M4 handled better than that Miata or I expected too much on Miata handling...
I think Miata chassis and suspension was still good. Just Miata body was not so rigid because it's a convertible... I feel a little bit loose when I push it harder on mountain roads...
I think 86 or BRZ should feel better... I need to rent one to find the result but I'm pretty sure 86/BRZ suspension is much firmer than that Miata... (I tested a 86 before)
To be fair I would like to mention some good points on that Miata....
1. I like its steering on highway... It's better than my GSF.... I think on highways my old modified G35S has similar steering feel as that Miata but my old G steering is even firmer and tighter thanks to very stiff suspension on G...
2. Thanks to Miata's very small size it becomes very easy to overtake any slow cars. I think Miata may be faster than my GSF during heavy traffic even though Miata has much less power...
3. Of course Miata MPG is MUCH MUCH MUCH better than my GSF and all my current Nissans...