Please be aware that many projector's light output will change as the zoom is used. Typically it will be dimmer at minimum (smallest image size) compared to a shorter throw set up with more zoom applied, even if the screens were the same size
. I have measured my light output when zooming to fill my 2.35:1 screen and also when using my Isco II lens. Both readings were very nearly the same. Now my set up means that I have to zoom back to the end stops when using my lens, so perhaps there is a bigger change at this end with my (older) JVC HD350/RS10, but it's just an example that using a lens does not guarantee a 33% brighter image as some seem to think.
You don't say what lens you intend to buy either. There is a cheap new lens being discussed in the 2.35:1 forum, but due to it's lower cost it doesn't have the full correction that better lenses have, so it requires that the projector it is used with has sub pixel adjustments for convergence, but your existing projector doesn't have this feature I believe?
Although I'm very pleased with my Isco II (bought for a good price used 2-3 years ago now) you have to view the projector/lens as a whole, so it might be that in your case you would be better upgrading the projector rather than buying a cheap lens. It also depends on how much 2.35:1 content you watch as of course for 16:9 your image won't be any better than now.