I've done a few random tests of 'hi-lo' deck results.
I won't bore you with details, but do these three tests, "flat betting", & you'll be convinced of the impact:
1. Take a single deck & remove all of the 5s & 6s. Cut away the bottom 1/3 of your deck. Deal 'pretend' blackjack hands (from the dealer's perspective). Follow all casino blackjack rules.
2. Do the same thing, except remove all of the Jacks & Queens.
3. Do the same thing, only remove all of the 2s, 3s, 4s, 5s & 6s, but don't cut away any cards, as you've already removed more than 1/3. Take "insurance" or even-money every time the dealer has an ace showing.
Over the course of 3 decks, see how you did.
You should find that you're ahead playing #1 & #3, and that you're down playing #2.
It was enough (plus, just the math of it all) to convince me to vary my bets based on the mix of cards remaining.
I play double-deck games, and keep a separate track of remaining 4s & 5s. This helps when you're dealt a 15 or 16 vs. a dealer 7-10.