The long cage derailleur should not be an issue when going to a smaller size big gear in the back (i.e. 30 or 28 vs. 32). Some people say that when using a long cage derailleur with smaller cogs in the back the shifting is less "crisp", but that most likely will not be noticeable unless you really were looking for it.
May I ask why you are switching cassettes vs. going to larger chain rings and keeping an 11-32 in the back?
Are you trying to avoid certain gear ratio jumps between some of the cogs, which makes cadence feel like it is changing more suddenly? If the answer is yes here, then an 11-28 or 11-30 can help. See below for the number of teeth each cog has in the cassette. Notice how the jumps on number of teeth are less for some cassettes.
Do you think an 11-28 will make you faster than an 11-32 with the same chain rings up front? If the answer is yes here, that unfortunately would not be the case as with all the cassettes the smallest gear in the back has 11 teeth.
If you want to go faster and can push the harder gearing without worrying about the gear ratio jumps, I would suggest going bigger on the chain rings and keeping the 11-32 cassette in the back. This way it gives you some options to still be able to climb very steep hills while being able to put power down when you are cruising on descents around 40 MPH + -
Note there I have 50X34 upfront and am considering as my next upgrade larger chain rings for 2 reasons.
I feel like I could benefit from them on flats after a bit more training with what I am running now. For me when I am pushing cog 10 or 11 without crossing my anaerobic threshold consistently at around 80 RPM, I will make the switch to larger chain rings while keeping my 11-32 cassette in the back. Right now, I can do that with cog 9 in the back so I have some room to improve.
On descents with the larger chain rings, I should be able to hit 40 MPH at the same cadence or so that I am hitting 38 MPH now. For me as someone who does not like high cadences other than when climbing that is a plus.
Hope that helps