Is that because of age, or a fault with V5 heads? Do WRX's of the same vintage suffer from this problem?

The real advantage is that the power stays higher all the way to the redline rather than falling over at 6000rpm with stock cams (even the v5/6 STI cams fall off above 6500).

Having a sizeable increase in midrange torque is another advantage of the better breathing cams bring to the party too.

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[CENTER][COLOR=#ff0000]Built Ej207: Semi-closed RA Cases - Wiseco Forgies - Manley rods - BC 272 cams - GDA Heads - billet f/whl - Vi-pec - WbO2 - 800cc Yellow tops - TD05-20G - 409Hp ath @ 1.75bar on E85[/COLOR][/CENTER]

It's a design issue that creates stress between the plug hole threads and the valve seat inserts.

My original V6 Ej207 heads (260,000km approx) looked like this after cleaning:



All plug threads were the same with the cracks visible down the plug threads to a depth of about 10mm.

Some builders re-use them if the cracks are less than 5mm in depth - I wouldn't - there is a risk that a valve seat insert could drop out and fuck the heads and bottom end.

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[CENTER][SIGPIC][/SIGPIC][/CENTER]
[CENTER][COLOR=#ff0000]Built Ej207: Semi-closed RA Cases - Wiseco Forgies - Manley rods - BC 272 cams - GDA Heads - billet f/whl - Vi-pec - WbO2 - 800cc Yellow tops - TD05-20G - 409Hp ath @ 1.75bar on E85[/COLOR][/CENTER]