How far can I span my cable lines without vertical support points - and will I have much sag?
This depends on the quality of turnbuckles, and the stability of your anchor points. If you are using commercial turnbuckles, steel wall plates (with 4 lags each), you can span 100'+ without vertical supports and still have very taut lines. However, if you have the ability to add vertical drop supports, it will certainly help take some of the load off the lines, and not require the lines to "work as hard." Keep in mind vertical supports will obstruct the sliding ability of your cage, so you would need to dis-engage each vertical support prior to sliding your net like a curtain.
More Indoor CurtainCage Cable Kits Frequently Asked Questions