So is it a good idea to replace the two exhaust 12cm tricools with 800rpm slipstreams? According to some sources they shouldn't be mounted horizontally but if I do so will they die quickly?
I like slipstreams although they shoudlnt be considered a long life fan, sleeve bearing fans dont like horizontal placement and according to reviews it might lower the life of the bearing... how much is hard to say, for horizontal placement search for Scythe Gentle Typhoons, they use a real bearing. There are other options of fans that you might want to consider, like Nexus BASIC D12SL-12 120mm Case Fan
or if you have a higher budget, Noiseblocker NB-Multiframe M12-S1 120mmx25mm Ultra Silent Fan - 750 RPM - below 6 dBA
(in my experience its more like 850rpm).
Also how much quieter are the 800rpm slipstreams? Do I need a fan controller?
Hard to say, each setup is different, depends a lot into what other components you will have. When ever i built a PC wihout PWM fans i do recommend to use a fan controller, so in case you dont like or even dont need the 800rpm speed you can lower it to your personal preference/need. But you can add this later if you feel you need it, you can also use cheap single fan controllers like zalman fanmate 2, etc. A fan controller will also allow you to go with higher rpm fan and lower it to your personal preference, and you still can ramp it up if its summer or if you were to do anything that will demand much more cooling.