You underestimate how crappy oem Honda speakers are. A cheap pair of Pioneers in the front doors would be like night & day. But to use the factory unit, the replacement speakers would have to be efficient. I say all this because, let's face it, the Insight is a hollow tin can. (See old threads about sound deadening)
Because the Insight is no long made, we tend to have the most traffic around here with folks with problems. I might suggest you check out Fit & Civic forums
for posts about adding amps to the factory NAV unit. As long as they are the same vintage, those people's experiences will apply to the Insight. Civic guys are gonna be way more into that than the typical Insight cheapskate, I mean driver.