need advice on motor height !!!!

[kbaudoin]

i have an 18ft team persuasion custom aluminum boat that i had a gill bracket built for because i bought a 20 inch shaft 175 evenrude etec motor, someone said if i trimmed all the way down and was throwing a rooster tail on each side of the cavitation plate that my motor needed to come up some more to gain more top end , my boat weighs about 1600lbs and does 49 mph at 5700 rpms they say i should be running in the mid 50s to low 60s,i will have to buy a jack plate to bring it up higher,and have the prop worked to bring the rpms back down and keep the top end , IS THIS TRUE??? ANY HELP WOULD BE APPRECIATED , THANKS