There are options beyond the systema 1 joule for a spring. I prefer the guarder line of springs myself. The guarder sp90 is a decent spring and would give you something in the area under 350. Like someone else stated, a larger battery isnt always a good thing. Having to large a battery with to much juice and to weak a spring could possibly cause alot more wear and tear on the internals due to such a high speed.
Heres a another link to a list of springs, the first chart is what you should be looking at for under 350.
http://www.mechbox.com/site/inside-t...son-chart.html
Like haggis linked earlier, here is a link to the JG HK 416 review, which is actually writen by myself.
http://www.airsoftcanada.com/showthread.php?t=50930
I do believe the 416 reviewed in the review is a generation two in the 416 series. Systema clone mechbox, 417 style stock, well you get the idea, all the info in the review.