In universe, the easiest rationalization is that Kira simply doesn't know what she's talking about.
If we assume that, though, then Dax also "doesn't know what she's talking about", and Dax is the science and technology expert!
Besides, Kirk also commented on the dangers of going to warp in the solar system, but judged the risk worth it to intercept V'Ger as early as possible. I also recall the Borg themselves dropped out of warp when they entered the solar system, and so did Enterprise
when they were in pursuit. So, there's more than one
instance mentioning or suggesting the risk.
Frankly, I like my idea to explain these instances, but that's only natural.