If you want to keep the Magnaflow midpipe you have, then stay with shorties. Any brand of shorty header should mount to that midpipe no prob, as they are all designed to fit in stock locations. If you go lt, you will need a short midpipe by the same manufacturer of the headers. LT's from one brand to the next end up at slightly different lengths and slight variances in overall fit. So basically, a midpipe from SLP is designed to fit with SLP lt's, and likely won't bolt right up to anybody else's lt's, at least not without modification. I suppose a decent exhaust shop could also modify your current midpipe to fit lt's, in theory anyway, but you'd really have to find and talk to a local shop about that one.
In terms of hp gains, shorties are basically worthless on our cars; usually about 5rwhp and similar tq numbers. Not much bang for the buck there. Probably make exhaust tone a little nicer though.
LT's are a horrible install - or figure probably somewhere in the ballpark of 8 hrs shop labor if you don't diy. Another consideration (and do your homework here, as they all fit differently and some better than others), is if you ever find yourself wanting to drop the transmission for clutch replacement, you may need to remove at least one of the lt's in order to be able to actually remove the trans
. BUT... You will be rewarded with a reasonable gain, usually around 15rwhp.
Lastly, I just noticed where you live, and got some bad news for ya.... Most headers do not carry CARB certification, and the few that do are exponentially more expensive than those that don't. No CARB E.O.#, no pass bi-annual smog testing. They're dicks here in CA. And to my knowledge, there are no lt headers that have CARB certification; has something to do with how the upstream O2 sensor is mounted in a different location from stock and also losing 2 of the 4 factory catalytic convertors; our laws here dictate that anything pollution control/engine management type sensors remain in stock configurations/locations. Now, if you have friends in the right places and smog is no worries, then well do whatever you want.