Something worth keeping in mind, is that the cost of buying a set of RCF/GSF manifolds from Lexus is going to exceed the cost of buying a set of stainless ISF performance manifolds form the USA, and shipping them, and paying import taxes.
It's worth thinking about as RCF/GSF manifolds won't last as long, won't add any power, and won't add to the vehicles resale value.
To be clear I'm referring to the catless versions (as the ISF can still pass MOT with just the rear cats, it was heavily over catted form factory to meet JDM emissions) but there are catted versions available too.