Somewhat correct.
The baddest thing you can stick on the back, IMO is a Mercury #6. Designed by Carl Keikhaefer, rated to handle 1000+ hp/ 1000+ TQ. Many people are running more power through one, the RIO ROSES Fountain (V bottom kilo record holder at over 170mph) had twin Sterlings putting out over 2000 ft/lb. The standard bravo will handle around 700, if you baby it. The Xr is a beefed up Bravo. Strait cut gears, thicker upper case, a lot harder to break. There is a difference you can hear between a standard Bravo and XR when running down the lake. A loud buzzing sound.
The Bravo 3 also uses counter rotating props.
No one marinizes any ford engine. PCM used to use the 351, for Nautique, but right now PCM has 2 350's (one is 330hp, 430 tq) and a 6 liter (375hp, 492 tq).
On speed, no it is not RPM. You can change the props pitch. Pitch is theoretically how far it would move the boat forward with 0% slip, in one revolution. Gear ratios in the upper unit of the drive can be changed (Bravo one can be 1.36, 1.50, 1.65). The smaller the number, the more power it takes to spin, so a smaller prop will have to be used. The higher numbers accelerate better, but must have a bigger prop.
You control the RPMs by putting a prop on with more pitch.
Most marine engines are built to run 5000-5200 max RPM.
Polishing weakens the blades, meaning more slip.
Fun fact, the standard Bravo 1 retails for a little under 9k. The XR is about 13,500, and the Number 6 (which is not a shiftable drive like Bravo's, and price does not include the 15k transmission) I cant remember retail cost, but my dealer cost is over 35k.