Case open is worse because your are eliminating any air flow. However, I think that temp is way too high. I had a heavily overclocked 8150 under water and would hit that temp. Yes, its water but I was also at 4.9 with massive voltage increases. I would still stay under 70. Please don't compare intel to amd for temps. Trust me, not remotely close at all.
You don't want to go above 65c for the most part with that cpu. Since your using a decent cooler, I would think your temps should be around mid 50s and even lower. A stock cooler on stock clocks shouldn't go about 60c in prime95. Just looking at some of my notes.
Prime95 is not real world results IMO, While I get why some use it, there's little chance for the average user to push there system that hard to get to the temps Prime pushes it up to. Keep an eye on your temps. If you start crashing on heavy gaming or rendering then you are unstable at some level, but it will unlikely be from heat don't get hung up on Prime95 results unless your a purist.
I have a 8350 as well and it can get quite hot on stock (bout 63 tops on prime95) but that is with a hyper 212 evo in push/pull config.
i'm not sure if changing voltages from the stock would be a good idea IMO but that's just me. Also I believe I have heard the max temp on this chip before throttling starts is about 65 (someone please correct me if I am wrong).