as far as benefits, pretty much anything anywhere in a rolla that stiffer is goin to be a bonus (as long as it doesnt add too much weight). a half cage made a huge difference to the handling of my rolla when i had it.
the main seams are the ones which are under the most stress, and connect to the more important parts of the car. i.e strut towers, wheel arch's and chasis. weld these to each other and you should get some positive gains from them.
i recomend searching through old scholl rally builds and such where this was done a lot and see what they did.
in reguards to legalities, if the engine bay is painted i challenge any cop to pick out that the bay has been seam welded. i dont think its illegal any way, just say you bought the car like that, and try to talk your way out of it if they pick it. i talked my way out of a weber carby, gatorade being used as a catch can, and twin tips poking about 1/2m out the back of the car. just depends who you talk to...
-dan