According to IBM'ers who have commented on this, DB2 works very well on Solaris, about the same as AIX. DB2 for OS/390 is a whole different animal.
DB2 has done very well on TPC benchmarks compared to other databases on Unix, Linux, and Windows. The benchmarks are sponsored by the hardware vendors, and Sun does not participate in the benchmarks, but I doubt there is a big difference.
As you mentioned, performance is complex issue, and a detailed discussion of the situation is required. However, when there is an obvious difference in a load operation, it often is related to whether one of the databases is logging updates, or other logging parameters that need to be tuned.
The other thing that trips up many people is the very low default bufferpool allocation that DB2 uses upon creation of a database. Run the Performance Wizard from the Control Center, and allocate at least 1/2 of available system memory to DB2. The more memory the better, however if Sybase is running at the same time, you need to make sure both systems have enough real memory.