As a person with first hand experience on Cisco and HP Access Points, I have some disappointing news for you. There is no comparison between the Cisco and HP wireless gear. The HP stuff is repackaged Calubris Netowrk gear and it is kludgy, at best. I really, really, really have a shudder of fear when client select the HP stuff. It simply doesn't work well. The Cisco gear using a WLC (Wireless LAN Controller) is the absolute best I;ve ever seen. It's a breeze to install and performance is great.
Keep in mind that when using a mesh, the bandwidth goed down 50% at each hop. So, if you have 54Mbps at the first access point, the next access points are limites to 27Mbps and the next at down to 13.5. This is under PERFECT conditions. Your distance of 300 feet is too far. I would recommend a mesh being no more than 150 feet outdoors. Preferably 100 ft.
There are limitations to wireless LANS and in the end, your best bet is to create multiple meshes, each with a hard drop back to a switch. You can get underground CAT5 pretty cheap. Rather than running the whole mesh off one drop... have 3 or four of them out to the field and set up your access groups accordingly. But seriously... I would just lose the HP gear. It's so bad. I even yelled at the HP rep... just my $0.02.