I own an IS401. We crossed the pond in 2012 and spent 2 years in northern Europe living aboard
. We are now in the Med via the Rhine-Danube route
. We saw 40+ knots between Bermuda
and Horta for 3 plus days avoiding a tropical storm (10 or 12 days at 30+). I never felt threatened nor was slapping a major issue.
from slapping under the table can be minimized by filling the sub-floor with foam and there are numerous other minor improvements that can make her even better. As stated, when loaded for cruising she is not fast but few are when you add a lot of weight.
I strongly suggest you visit the Island Spirit facebook page to at least read the entries. It is mainly focused on technical stuff. https://www.facebook.com/groups/14114077331/
You might also want skim our blog. SV Ostinato
It is not very technical but will give an idea of what we have put her through. Good luck!