Svend, The cap weighs roughly 3.5 grams. So, yes it will allow more arms to work with the cart. The Cap with Cart weighs about 12 grams, the EM of a RB300 is 11.5, so it should put the resonant frequency around 10-11 Hz.
In prototyping the Cap, I worked very hard at using brass in stead of aluminum. However, I just couldn't get the weight down enough for it to be reliable. When they get too heavy, the life just gets sucked out of the sound.