I was wondering if anyone is specifically running a welded diff on their 3is. Ive known people who have successfully installed an FRS differential on an IS250 but there seems to be no LSD options for the IS200t.

 

Welcome to the forum!

I doubt you'll find anyone running a welded diff. on an IS...it's just not that kind of crowd.
And yes, the FRS LSD diff. does fit the IS250.
In later years of the IS they introduced an LSD on the 350...you may want to look into that as a possible swap.
You'd have to see what the gear ratio of that diff. vs your 200t is though.

 

From what I can gather the 200t RWD has the 3.13 ratio like the is350 RWD; so you should be able to swap in a 2018+ OEM LSD unit, or get an aftermarket diff if you want something more aggressive.

To welding the diff, haven't seen anyone doing it to a 3is, but I have seen it done in 1st gen/IS300's for drift duty - it's not rocker surgery, if you've welded a diff in another car (350z for instance, those tend to get that treatment), you can weld the diff in the 3IS.

I wouldn't suggest it however; especially not on a street driven car, as there are risks associated with welding a diff (or running a locker or spool on the street), the proper way would be to go aftermarket and get a proper 2-way LSD for street & track duty, or a 1.5way LSD for street duty if you want something more aggressive than the OEM unit.
A 2way can still bite you in the A$$ if you aren't paying attention, much like a locker can under deceleration on a turn. I'll personally be sourcing an OEM LSD from a 2018+ as my wife drives the car as well (she took it today actually) and she does not possess the skill (or care to learn the skill) of dealing with an aggressive diff in adverse conditions.

Anecdote: I used to have a Fox Mustang back in the day, sub-3,000lbs with full interior and AC, 300RWHP, 3.55 gears, Moser 31 spline axles with what would be called a "2way" LSD today (wasn't called that back then) and it was a good balance for a primarily street driven car that saw track duty on the weekends, without going all "banana pancakes" with a locker like my drag buddies had; I wasn't into drag racing as much as I was circuit racing, and I drove my car daily hence the interior & AC.

I don't want to get on a soap box and rehash different differential styles, but suffice it to say, figure out your intended use and go from there.
my decisions chart below
drag only - properly welded diff=spool>locker>2-way LSD>open
primarily circuit - 2way LSD>1.5way>1way>locker>open>spool=properly welded diff
primarily street - LSD (pick you poison1, 1.5 or 2way)>open>locker>spool=properly welded diff

By "properly welded diff", I mean a certified welder/fabricator, NOT a buddy with a harbor freight welder, welding it over a couple beers in an unventilated garage.

I tried to stay off my soap box, but I still put a foot on it, sorry for that.

 

daily driving with a welded isn't awful, but why are you trying to weld it in the first place?
if you're going to buy a diff to put onto the 3is you might as well get an LSD at that price point.