If a model is set up for SS lines and tip-weight balanced, if you just change to Spectra you 'might' have to reduce the tip weight. Just fly the model w/ Spectra and you will know.
60' 0.018 SS lines might produce say 1/2 to 1 oz of static weight on the inboard wing. Compared to SS, Spectra will produce almost no added static weight.
One other thing I have experienced. Spectra does have a little less drag in flight compared to SS. I have noticed a slight increase in model pull with Spectra which makes sense to me.
Spectra also has a little more line stretch compared to SS. Not much, but it is there.
But if sport flying, the above is really not an issue. But may be an issue for a serious competitor.
Just my 2 cents.