Sorry - not quite.
If he were taking Plain-Jane (??? ???) Thunderwolf Cav, then at Strength 5, he would get Strength 10 with a power fist.
As an upgrade to an independant Character - You double first, then add the bonus (actually shown in BRB - page 7 - in box marked "Multiple Modifiers - exact strength that a Wolf Lord with a power fist on a Thunderwolf would be.). As the Upgrades page shows - Thunderwolf Adds to the strength value - does not replace it.
One of the times where you're better off using the "Normal Guy" instead of a character!
<WHOOPS> Ignore me! Thought you were talking about the lord with the Thunderhammer - didn't see the 3 TW Riders at bottom of the list!