If you had a problem that was solved by putting IUSR_* in the power users, it typically means that the installer was unable to grant the necessary privileges to IUSR_...
Typically this is done with the command aspnet_regiis.exe -ga <USER>, e.g.
If you want to do this and remove IUSR_* from the Power users group, you might want to do the same for two more users: IWAM_<Computer> and ASPNET. These 3 users are used in different contextes by IIS and all should have privileges to run ASP.NET under IIS.
This applies for OutSystems and for any ASP.NET 2.0 application you might want to run under IIS.