Thanks for your bug report. There would be no difference between 32bit and 64bit applications in this context.
Transfer speed: mIRC is a single-threaded application and intentionally makes the GUI take precedence over transfers to ensure that the GUI is usable. In practical terms, this means that transfers will necessarily be slower. This cannot be changed at this time.
Regarding corrupt files: I have not been able to reproduce an issue with this so far and do not recall anyone reporting such an issue recently. I cannot think of a reason why an increase in your connection speed would result in this. I have tested multiple simultaneous transfers with a range of files sizes from a 10MB to 10Gigs over a LAN and was unable to reproduce an issue with corrupt files.
Are you using anti-virus software? If you disable it, does that resolve the issue?
If you install a clean copy of mIRC to an empty folder in your Windows Documents folder, using the "portable" option in the installer, with no scripts or addons, and then test this copy of mIRC, do you see the same issue with it?