We currently have a WLC 4404 with 1142 AP's. We plan to upgrade to a new WLC 3504 and 1852 or 2802 access points.
Assuming the wireless design is the same and that it is configured properly, I am curious what are the main benefits we will see with the new AP's vs the old AP's with regards to performance and security. Also, will client capacity possibly be increased with new APs?
Assuming the wireless design is the same and that it is configured properly
This assumption is wrong.
There is a world of differences with codes running on 7.X (and earlier) and 8.5 (and later). Next, different AP models have different radiation patterns (aka "footprints"). Upgrading the AP isn't as simple as one-to-one-replacement. A wireless site planning needs to be conducted so one can understand how many APs are require and where to put the APs.
I agree with Leo, it is best if you can do a wireless site survey to see how RF in your environment with those newer AP. In general it should be much better than 1142, but it must be verify.
I am curious what are the main benefits we will see with the new AP's vs the old AP's with regards to performance and security. Also, will client capacity possibly be increased with new APs?
In general performance point of view there is a big difference of two AP models. 1142 is supporting 802.11 a/b/g/n standard where as 2802 supporting 802.11a/b/g/n/ac (note that 11ac is newest addition). Most of client devices supporting dual band & in 5GHz, they may support 802.11ac, please check this link & filter .11 column to see all "802.11ac" supporting client types.
With 802.11ac, you will get improved efficiency over wireless as a given frame can use higher data rates (given your RF RSSI/SNR is good enough to achieve those higher data rates).
From AP capacity point of view, 1142 is 2x3 with 2 spatial stream where as 2802 is 4x4 with 3 spatial stream. This gives theoratical max of 300Mpbs for 1142 where as 800Mpbs for 2802 (with 40MHz channel width, which is most practical in enterprise. Yes, you can get higher throughput by bonding 4 channels - 80MHz, but it is impractical to have enough channels in enterprise). Pls refer below for those data rate values.