I tried a longer stem on my mtb too. it's 70mm, which is not "old school dirt roadie" long, but it's 20mm longer and certainly longer than what is trendy. I went on a 10 mile trail ride and didn't die. I usually don't bother checking data on Strava, but during that ride I absolutely crushed 9 PRs on a bunch of segments. could be a coincidence. time will tell.