You want to keep the shroud on, it creates a tunnel effect to blow the hot air out the back of you PC at an accelerated rate. Try replacing thermal pads and paste. Also, I have 120mm fans mounted in the front of mine to blow even more air through them and a 40mm fan mounted on the top exhaust port sucking air out. I lost around 20° C on my exterior cards and 15° C on my interior cards. Just make sure when you add fans to a cased PC that you create a nice flow through the case otherwise you wont accomplish anything.
More fans does not mean cooler tempsit really all depends on your situation but if you have open air with proper cooling shroudless is cooler.
for instance I have a rig that has 5 reference 5870s in it, 2 of the 5870s have the shrouds off because I bought replacement fans for it that were just slightly too big
this rig is open air with plenty of cooling (open window in winter/ac in summer)
the two 5870s with no shrouds are running cooler than the other 3 5870s with the shrouds intact
and I can prove it!
this is fresh data from that rig
GPU 0 and 1 are the ones without the shrouds, I have all fans at 50% and as you can see they are running much cooler - of course if you had these packed inside of a case you would have a different story all together but then again you would never be able to pack 5 5870s inside a case to begin with
GPU # En Status Temp Fan% GPU Clk Mem Clk Volt Actv MH/s avg MH/s 5s Acc Rej Harderr Util Intens
0 Y Alive 58 3307 50 900 180 1.175 99 412.06 411.64 8433 58 0 5.65 9
1 Y Alive 48 3374 50 900 180 1.125 99 411.87 411.74 8451 47 0 5.66 9
2 Y Alive 77 3645 50 900 180 1.125 99 412.15 413.98 8447 55 0 5.66 9
3 Y Alive 65.5 3149 50 900 180 1.175 99 409.75 411.51 8245 35 0 5.52 9
4 Y Alive 73 2331 50 900 180 1.163 99 412.06 413.2 8257 37 0 5.53 9
