Well mine has 195/50/15 tyres fitted, Can you confirm this is the size mentioned in your handbook?
The same reviewer also said he'd changed his tyres to a lower profile /45/ and found the ride and handling improved. While I can believe that the grip improves, surely the ride gets bumpier? I would agree if you had a like for like consturction/materials, going to a 45 series tyre, the ride should not be as good, but it is more likely the construction/brand of the tyre was changed, can't comment much further without seeing the review.
And doesn't changing the profile mess up the gearing and speedometer and so on? Yes
And how far away from the recommended tyre spec on stock wheels are you allowed to go before it gets dangerous and/or illegal? difficult one, probably more of a question for insurance, anyway I would not recommend moving away from the original design diameter on the stock tyres, you can still change things around for instance
195/50 R15 (577mm) = 195/45 R16 (582mm) = 205/40 R16 (570mm)and so on.