There is no FAR that prohibits a light sport pilot from filing and flying IFR. Pilot must be instrument rated and current. Aircraft must be equipped appropriately. Pilot must comply with all light sport restrictions, ie the 10K altitude limit the poster mentioned.
I have same setup as you. Information given to you is correct. I installed an RC Allen 2610 - very, very easy install. Works great. Got rid of vacuum system. This is in a Mooney M20J. Buy the one with the new yaw indicator, install in turn coordinator location.
I used own one and climbs on hot days were a problem sometimes. Couple of things, make sure winterization plate is off the oil cooler, check baffling carefully to make sure its good. There was a good article on this by Mike Busch in the Dec 2020 AOPA mag. If it heats up above 380 level off...
I know the exact place you are talking about. Was really bad in my buddies Mooney, this annual he removed antenna cleaned up bonding, re-installed antenna and replaced coax with the new good stuff. Eliminated the problem completely at the Issaquah towers area.
Most of the time when I go to a new fbo, I give them a call, get prices, etc, ask about any special procedures. They ask about what time you,ll show then we say see you then. Call em first, prevents surprises, maybe expensive ones.