I can't remember when the VMware PV controller driver was included in RHEL - maybe not until 6. If it's not there I assume it gets installed with VMware tools so make sure they're installed before attempting to switch.
Fundamentally things like the PV SCSI are tinkering around the edges though - if you're asking for more sustained IOPS than your storage can provide then its response times will lengthen. Changes that affect SQL execution plans (and there are many), whether application or database configuration inflicted, then have an impact on server resources (most typically storage).
Whilst I don't think you described the context of this performance problem too much, if it's a production system that has been running happily for months (or maybe a year or two given that you're on ESXi 4.1), the first question is probably "what's changed?". If it's a new application version but roughly the same amount of functionality and business, most likely the application/database configuration (like stats) is at fault. If there's a load of new business and processing, perhaps the storage is now undersized. Finally with shared storage I'd suggest making sure that something else outside your control hasn't changed, e.g. that there aren't some new VMs using the same physcial disks as you (perhaps after a storage admin did some reorganisation).
Good luck!