I suggest you add atleast a metal spring guide. (E1s only have metal bushings)
Also, get a new hop up bucking. These work great with my E1 G36C. http://www.airsoftgi.com/product_info.php?cPath=31_170&products_id=1956
I don't know why the reviews are so bad for it, my silicone bucking works great. I used to have to turn the hop up almost all the way when using .25s, now I barely have it above no hop up.
The rubber bucking didn't fit, but I could probably get it in there with some lube. The silicone one slid in easy and works great. Plus it comes with two fishbone spacers which is nice because spacers are really easy to lose.
I'm estimating you will get about 180 feet of range. Around 150 effective max. (A very rough estimate)
Why are you putting in a weaker spring? You would still have really nice rof with a 9.6 with the stock spring. If you did that, then you wouldn't have to send it to the shop.
Also, you can easily install the barrel and bucking for yourself. (if the shop will do it for no extra charge, them let them do it)
But it is really really easy to install a new barrel and hop up.https://www.youtube.com/watch?v=vNFZw8IKVYEhttp://www.airsoftretreat.com/reviews/showproduct.php?product=118&cat=49
these will show you how to do all that. (the youtube one is really the only one you need to follow, the other one is for taking apart the WHOLE hop up)
hope this helps