I have GT 2170s and have Sorbothane heel inserts in (small ones just for the heel part only, not reaching to midfoot) - I have no problems with stability and my footfall feels solid and stable (landing midfoot, more or less).
Are your Orthaheels the full length blue sports ones with the firm arch support? I have the full length blue ones from a while back when I used them with my old shoes - I just put them in my GT2170s and they didn't feel right at all, they pushed my feet upwards and outwards and made me feel bandy legged. But it depends on the shape of your feet I suppose, they might be ok with you. The person in the Asics store assessed the GT2170s as being ideal for me, so using the Orthaheels with these would change my running style too much. The heel cushioning feels fairly minimal (though mine might have got impacted down by now) so I think that would be ok (if they replace the original somes rather than on top of them).