Can a hybrid bike's stem be slammed to make it nearly as fast and powerful as a road or cyclocross bike?
More aggressive positions are thought to improve power transfer. That means a better workout when doing HIIT.
Fit will always be a compromise between:
Very low positions can be great for aerodynamics, but they can compromise comfort and power. If you train in an upright position, suddenly dropping your position your bio-mechanics can become compromised, which will impact your power production. Similarly, if a position is too uncomfortable you it can be hard to maintain, which will impact amount of training and by extension sustainable power.
Low == Fast is an over simplification.
You need to start with your end goal and work backwards. If you only goal is power during High Intensity Interval Training (HIIT), then buy or borrow a power meter and experiment with position until you max out your power numbers.
If your goal is time trial then you will need to work on balancing all three components with a focus on aerodynamics. For example, if you have trouble producing power in an aero position, you will need to work on your flexibility and slowly transition your position during training.
If your goal is road racing you will need to determine what type of road racer you are, then that will indicate the compromise between aero/power/comfort you will need to focus on.
According to the following article, it looks like we need to select a stem with an angle and length that makes the handlebar height 5 to 10 cm below that of the saddle. https://www.mec.ca/en/explore/fitting-your-bike
Those are very general guidelines for people that have limited cycling experience. Your mileage may vary. Some people simply can't ride a large saddle to bar drop, but can still be competitive with a higher handle bar height (there are many ways to try and whittle down your frontal area).
If I use the right stem so that the reach and handlebar heights are similar to those of road bikes, would it work? The stem may need to be longer than you can find in stores because there are no hoods to extend the reach.
There are many aftermarket stems at a variety of lenghts and angles that open up a very large fit window for most bikes.
You can play around with the calculation here: https://www.bikegeocalc.com