i've heelstriked my way to several sub-50 4x400 splits.
i worked with a power 5 sprint coach on my form in general, and ultimately we decided not to change my footstrike because
1. my cadence is "elite", even as a heelstriker
2. to forefoot strike, i would need to shorten my stride. i cannot afford to do that, as i am much shorter than the typical 400/800 runner
3. no injury issues related to footstrike
how fast you are is a simple calculation of stride length * cadence. if a slight decrease in one area can result in a huge increase in the other, it may be worth changing your stride. but it depends person to person.