the panasonic television you mentioned uses an "ips alpha" display which is only for televisions. keep in mind that the screens used in televisions and monitors often vary a bit in their technology, you really cannot compare them as apples to apples. i've never heard of ips-a before and theres a lack of data to compare it to normal ips panels so i'd be wary about if it really stands up with the rest of the ips crowd in terms of quality. not saying it is worse, but remember even e-ips shares the ips name.
with computer monitors you're pretty much stuck with the normal 6 panel types at 60hz in the usa or 120hz in the usa if using dual-dvi (or maybe hdmi) on tn panels.
you stated that the dell monitor is around 3" too large physically. keep in mind that most of the screens in your ideal size range are going to be 1920x1080 and will look much less sharp in everything except for maybe native 1080p content such as bluerays. for reference a 27" @1920x1080 has around 2.1 million pixels while the dell 30" @2560x1600 has around 4.1 million.
as far as input lag is concerned... i've never really noticed any on either my older 16ms 60hz S-IPS response time monitor or my S-IPS 60hz television with any type of game be it fps, rpg, etcetera. though i have heard that 120hz televisions can generate alot of input lag due to the processing unless they are in "pc-mode" which is something you might want to think about before buying a 120hz or 240hz television.
as for what you should buy... you still have to make a choice as there is no perfect product in existance.
once you make your choice its time to do a little research.