Now its clear
First, 1-100 should be done in 1 day...ok, very lazy mode in 3 days(yes, with standard items even for iss).
Regarding ISS-s, at current game style ISS Hierophant is the only ISS that matter in a party. Iss Hiero main features is POM buff and ability to AOE debuff.
Compared to ISS BD, ISS Hiero might look like he is much stronger and for a live player might be true...but, ISS BD can solo afk macro with all his buffs up and that is a biiiggg plus for BD. Also its true that ISS BD cant aoe debuff mobs-so for your Feoh is not good.
Regarding how powerful is an ISS...well, im ISS BD-105 former ISS Hiero- damage setup and I can solo farm in all areas, k99,Compass event Instance etc and hell, in IC mobs dont even scratch me and I dont have uber items. Not too bad for a poor iss like me.
Too @MalumInSe its a little bit slower but you get all the exp after a day of grind you get almost same exp.
In the near future situation for Enchanters will be very diferent, ISS Dominator will be the only ISS played...they will get POM buff for clan members...and he can cast out of the party.