Sometimes its just that the client's electrics are super sensitive, so not really your fault.
It should be incumbent upon yourself to keep your equipment properly serviced, and if you do start tripping a client's electrics then it is probably time to do some more servicing of your equipment.
That said there are often ways round this problem. For example, plugging-in in differnt rooms/floors can help minimise a potential problem as you will then be distributing your electric load across different rings. Avoid using adaptors and plugging 2 plugs into a double socket. Kitchen rings are generally higher rated as well. If push comes to shove you can always turn off your heater element and just run the pumps and vacuums and if you are really struggling then asking to turn off some of the clients equipment may allow you to get a job completed.
Bottom line-adding a booster vac to your set up really shouldn't be the "straw that breaks the camel's back" when it comes to tripping clients' electrics.
And for the record I have the 400 psi with in line heater and booster vac-so I hope you can see that my answer comes from experience.
Rog