So I just tested in single player vs server and the jetpack performance significantly differs.
In single player I can press forward while thrusting with space to achieve some modest forward momentum while continuing upward. If I hold shift as a modifier, I fly perfectly level with the same modest momentum.
On our server with sync 20 where I am on the same LAN segment (no Internet lag) pressing space moves upward, and pressing forward achieves no noticeable forward progress. If I release space, I can push forward and alter my trajectory while falling by a small amount. Holding shift triggers a powered slow descent, but again no forward motion.
Edit: Other reports
http://www.reddit.com/r/feedthebeast/comments/2absst/rotarycraft_ethanol_jetpack_only_goes_up/
We consistently run at 20 TPS, but I have seen where if we lag to 13-15 TPS that the jetpack stutters and is very scary to use. All our players with RoC jetpacks use hang gliders for forward progress.