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A car is worth whatever you are willing to pay for it. The KBB, black book, edmunds, etc. are just guides. The most important thing is you are happy with your purchase.
I agree with the ISF not being that fast, a few mods will help but like anything else you get use to it and will want more power...
Stock 0-60mph in ~4.5 seconds? Stock quarter mile in ~12.8 seconds? Stock tire ~70mph slalom speeds? A constant pull all the way up to 160+mph? For a stock road legal car, this is fast by any definition of the word. Any sort of performance advantage a competing car may have (C63, M3, RS4/5, CTS-V etcetera) is marginal at best in comparison. You would have to upgrade to a six-figure supercar to get an obvious performance difference. Your sense of perspective is a bit off.
The price seems a bit high to me. But with that said, you can't take the money with you when you're gone, so you might as well spend it while you're here.