I don't know if the radicals really are underestimated. They may not be very popular, but they stand a fair chance against GT Pro cars, and I don't think anyone's ever disputed that.
Around Nordschliefe in Time Trial mode, I found the Radical SR4 252hp had little trouble keeping up with a GT1 Aston Martin DBR9. In fact, there's a lot of places where it's actually faster. Loads of grip, and a terrifying power-to-weight ratio. But I've never seen anyone suggest otherwise, so I don't think they count as underestimated.