Informix 5.07 to 7.22

I am looking for any advice to prompt moving from a 5.x system to a 7.2x system. As I understand it, the 7.x system will decrease the performance of the server unless I am running multiple cpus. If this is not the case or given a multiple cpu machine, I am looking for a strong argument for or against migrating to 7.2x. Please send me whatever thoughts you may have on the subject. I am looking at a large system 300+ users (not an enormous amount of data ~3GB). I appreciate the improvements in the backup facilities for ONLINE. I also appreciate the increased functionality. I have to weigh this against the case of considerable QA as well as some code changes. I also appreciate that this is old technology: does anyone have or know of software that will no longer be compatible with 5.x? I am not trying to find someone to make up my mind for me. I simply do not have enough information inhouse to make a compelling argument for or against 5.x or 7.2x for an OLTP system that is performing relatively well. BTW, I did have a DSS machine with 4 CPUS and 512 MB of ram that runs like a dream under 7.2x. I am running HP machines using HP-UX 9.04 through 10.20 with Windows 3.1, Windows 95 and Windows NT clients. Any information would be greatly appreciated.