Yes, your X1 plugs will work in Professional as far as I know. The only time a problem occurs I think is when you have Prochannel stuff and only upgrade to Sonar Artist... which has no Prochannel (aside from the very limited Style Dial thing they just introduced... I don't think proper PC plugs will work in it).
There are of course extra goodies you get with Platinum vs. Professional but you can look at the version comparison page to see if those extras are worth the extra money... which you may have already done.
tl;dr...
Pretty much everything from X1 will work in Professional.
If I am mistaken then someone will have to correct me (scook?).