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The athermic windscreen only seriously affects models with the older SiRFstar II chipset. That includes the original GO (AKA the 'Classic') and the 300/500/700 models that came later.
They switched to SiRFstar III with the Rider and the One. All of the subsequent models have been SiRFstar III.
So you really shouldn't need an external antenna, though the windscreen does still attenuate the signal, so you may find that you get a faster lock and more accurate location tracking with an external antenna. Try it first. If you're happy without the external antenna, then don't bother getting one.
Currently in Horatio's garage:
2010 Laguna III Initiale.