Yes, you're both right - it will get unhydrous to that point, and it is impossible to do at home - because an ordinary gas stove simply doesn't pull that temperature. SWIM tried it twice, both times ruined the flasks (once, after all patience had gone from him, he took away the metallic grid from above (immideately above) the flame - and the flask's bottom melted. Another time it simply cracked as it cooled down.
Also - don't you think that to condense SO3 down from so high a temp., one would need to have a
very good condenser? It boils at 40 C!! If a cloud of it escapes into the room, ............... will undoubtedly happen
The more realistic approach is to heat NaHSO4, first it rearranges to Na2S2O7 , which is itself widely available, and then, somewhere circa 300 C, it'll give off SO3. SWIM didn't try it as he beecame pitious of his flasks
Antoncho