Yes, angle plugs can make more power, as long as they are moving the plug's firing tip closer to the exhaust valve, and yes, they can cause fitment issues with some styles of headers
Like anything else, it can be give and take. On most of my engines, the angled plugs make it easier to get to the plugs for changes etc.... with block-hugger headers, you normally have to ensure that they will fit properly ahead of time, depending on the design of the tubes for the end cylinders 1,7,2 and 8.
for the center cylinders, it make it easier because the plug is angled away from the primary tubes..... all a matter of preference really...
If a straight-plug head has the plugs moved closer to the exhaust valve, then it will realize the same performance benefit as an angled plug, but then you are getting the plugs closer to the center tubes which can make plug changes more interesting as well as increasing the likelihood that you may barbecue a wire boot.... Also keep in mind that if you are wanting to run larger tube headers, especially block-hugger style, the angle plug design will do a better job of keeping the plugs on the center cylinders clear of the header tubes, but may increase the likelihood that you will have interference on the end cylinders.....