I'm not sure how you'd do it with NMSDK, but if you were using CLI it'd be:
95th percentile from the last 600 seconds
dfm perf data retrieve -o filer1:aggr1 -C aggregate:total_transfers -d 600 -m value_at_percentile -P 95
mean value between 1pm-2pm today
dfm perf data retrieve -o filer1:aggr1 -C aggregate:total_transfers -b "2014-11-21 13:00:00" -e "2014-11-21 14:00:00" -m mean
At the bottom of each output you'll see the output for -m.
if you want to see the value for all the aggr on a controller, just remove the ":aggr1" part and it'll get the numbers for all the aggrs for that controller. This works the same way for groups, so if you group filers(or aggrs), then you can plug in the group's object ID after -o and get the values that way. I do that sometimes when I need to pull numbers for an entire DC.
As far as I know, each counter group for each filer gets its own file in the perfdata directory. You can verify this by typing dfm perf data list -v.