My recommendation is don't bother - the standard manifolds are equal to any after market extractors.
The exhaust back from the manifolds is a different matter - not sure of what is needed but replacing the entire system back with a free flowing system is very worthwhile. Not sure what you have now is good enough but spending money on the exhaust will get better gains than changing the manifolds.
Garry
REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
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