You really don't have to nerf Missiles so much as it is
Per latest (and earlier errata)
Each Field Gun requires a number of troopers equal to its weight in tonnage to operate, rounding up (to a minimum of 2 troopers per Field Gun). A platoon equipped with multiple Field Guns can only operate as many Field Guns as it can keep fully manned (any excess Field Guns are considered destroyed as the unit takes damage).
Just change that minimum 2 per Gun to a 3 or 4. Just look at the RAC/5 or an Ultra 20. They look powerful but aren't. And Ultra AC/20 gets one ton of ammo. 5 SHOTS. To be an Uber weapon you would have to stay hidden and do a point blank shot.
Even having a 30 man platoon doesn't give your field gunners much of a chance in most games. Besides, only a Motorized platoon gets 30 men. Tracked 28, and wheeled 24.
Besides and alteration of minimum crew size your could also make sure that Missile field guns are restricted to Mechanized infantry only. Fluff it up to a matter of trailer size compared to the smaller autocannons (since most fluff has ACs using mostly smaller caliber shells).
Another suggestion beyond a restricted motive and larger minimum crew. Cluster penalties. It was brought up a few posts back, but since Field Guns don't have any (known) fire control the to-hit could remain unchanged just make sure to apply a cluster penalty.
There are ways to make missile field guns as scary as AC field guns w/o creating odd rules. Or breaking most current canon field guns. A HS requirement is not a good direction to go.
So, a project for all those involved. Break out two random map boards, create a LRM Field Gun company (3 platoons) based on the current field gun rules (including the changes in the 3.4 errata), setup a lance of mechs. Run a few scenarios with normal field guns and then LRM field guns and give a report on how it played out. Make sure at least one of those scenarios were on the open terrain maps, or a blank map to see how they fare with no cover. This will give everyone the best idea of how they will need to be tweaked for balance reasons. Sound good?