Would having the gears welded in the diff be the same as buying a performance 2 way lsd?

No it wouldn't. If you welded, the tires would turn at the same speed all the time. During a turn the outside wheel turns faster than the inside, so you don't want to weld the diff. A 2-way senses when a tire is slipping and sends more power to the tire getting traction. Plain and simple, you don't want to weld anything.

so a clutch type 2 way lsd only works when the inside, usually, tire starts to spin. right?

It engages upon acceleration and deceleration. The power is transfered to favor whatever tire is getting better traction. If you look online you can find huge descriptions of everything that is going on. Do that because I don't know all the specifics.

cool thanks