The factory-rated load capacity of the Porta II mount head is 5kg and that is the maximum load that any warranty claim would be covered for in the UK.
Field testing with loads of up to 20lbs/9kg with short length refractors has been successfully carried out by Vixen’s US distributor. Longer length tubes require that the tripod be upgraded to eliminate flexure.
Some dealers will state different load capacities based on their own and/or their customers' experience with the mount.
It is up to you to decide whether the product is suited to your needs should you wish to load it beyond the warranted load capacity of 5kg.