I've implemented the Change Set b07e65b198a7, to use a stored procedure instead Linq to perform search, but it seems the commonsettings.usestoredprocedureifsupported setting is ignored.
In which part of the code nopcommerce read this setting from DB?
Why do you think that it's ignored? It's used in ProductServer.SearchProducts(...) method. You can set it to 'true' or 'false' in admin area (configuration > settings > all settings).
This value is read from DB along with all the other properties of CommonSettings when passed into ProductService constructor. Please have a look at ConfigurationProvider class (\src\Libraries\Nop.Services\Configuration\). This class inits all properties of all the classes which implement ISettings interface (such as CommonSettings)