It is possible to aerialise (carry) the whole GT90 before making a shoot but it is far from easy! Everything has to be absolutely perfect in terms of stroke length, arc, tracking and haul timing. Also the rod needs to be up to it, very unlikely that you will break the rod, but, if the rod you are using is quite soft then you will need a very long stroke to keep it moving. When I am on form I can carry a whole GT90, but my best distances come from carrying about 83' (hauling hand to tip of fly line, excluding leader).
If you are playing about with that then do yourself a favour and stick something other than dacron backing behind it, dacron tangles badly.
The new GT125 goes further in terms of total distance for me, with that line best distances come from carrying about 85' and delivering from that. It is possible to carry more but for me, the shoot is not as clean so counter productive.
The SLX is not designed as a distance line, a cast of over 100' with that line is a very good shot but casts of 25 yards from feet to flies are very easy. You are relying on a lot of shoot with the SLX as it cannot be carried well due to short rear taper.
Hope that helps.
Cheers]
Ben