For my twin DG glider, the Type Certificate states:

The basic required equipment as prescribed in the applicable airworthiness regulation must be installed in the glider for certification.

The approved Flight Manual states:

As minimum equipment only the instruments and equipment specified in the equipment list (see maintenance manual) are admissible.

In the maintenance manual there are only two variometers identified:

Winter 5 St VM5 (daim. 58 mm) Certification No. TS10.230/14
Winter 5 StV5 (dia.80 mm) Certification No. TS 10.230/13

And I don't think these are just friendly suggestions. So you may very well be right, but good luck arguing the point with the FAA. From my own experience on a different issue, they are not easy to argue with, it is their way or the highway. I think once your A&P, or even the manufacturer, puts the variometer in the glider, it may be wise to use the one on the instrument list, even though it may be inferior. Having my standard airworthiness certificate jerked is not an experience I wish to repeat.