6" streamers doesn't sound bad. I get 4" from a 12kV/30 mA NST with .007 mf cap. My primary is out of tune.
9 kV, I've found, is just barely enough for a spark gap. 12 kv NST's are more sure. I can run two 1/8" gaps reliably with mine.
To answer your question, we need to know More--your total capacitance, number of turns in Secondary, etc.
Got to:http://www.teslacoildesign.com/#design , there is a formula somewhere for max output using your design.
"Don't look for any sense to life, you will only Find futility."--DFS
