I've never done a direct comparison between my modded 300D and my unmodded 300D (same exposure length). I never tried shots longer then 2 minutes with my unmodded camera because, at the time, I didnt have a light pollution filter that prevented saturation at 2 minutes, nor a mount that allowed me to take longer exposures without star trailing. I have both now

and I have pushed it to 6 minutes with my modded camera)

From comparison shots there seems to be quite a bit of difference.

W. Veil unmodded
W. Veil modded

Crescent nebula unmodded
Crescent modded

m16 unmodded
m16 modded

m20 unmodded
m20 modded

There is definitely a difference, but the decision is up to you. The modded cam will give up up to around 9x more response in the H-alpha regions.

If you arent satisfied with the umodded, you can always send your camera to get modded....and there is always a good second option---buy a cheap older model like I did with my 300D (you can get a body only for around $200) and do the mod yourself. Prices are always coming down for the older models and is always an option for the future.