The Requirements page for WAS Addon holds this information:
The Splunk Add-on for IBM WebSphere Application Server has no specific sizing guidelines for JMX or file monitoring inputs. For HPEL interface inputs, the add-on can concurrently collect metrics from twenty WebSphere profiles without performance implications.
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Taken from Splunk Release Notes
Version 3.0.0 of the Splunk Add-on for IBM WebSphere Application Server is compatible with the following software, CIM versions, and platforms.
Splunk platform versions 6.0 and above
CIM 4.2 and above
Platforms Platform independent
Vendor Products IBM WebSphere Application Server versions 7.0.0 - 8.5.5
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