I have tested CPU temps on unopened Xbox's of all versions and not seen any real difference after changing the paste.
If anything maybe 1 or 2 degrees cooler and I have tried Arctic Silver 5, Prolimatech PK2,a copper based paste, as well as liquid metal pads.
And we are talking about a 733 Mhz CPU not a 3 or 4 Ghz in a similar size package, The biggest drop I got was a new heatsink and that was about 5 deg celsius. it was 4 for silver and 5 for the black paint.
Put if you feel better after changing the thermal paste go for it.
As for the speaker cable I did blind test with speakers that sold for $10k and a $5 k 2 channel amp and using cheap cable that most people use it sounded excellent to my wooden ears but when the cable was changed to $2k worth for 10 mtrs holly molly the music came alive.
Rant Off.
Cheers
SS Dave
Soft modding is like masturbating, It get's the job done but it's nothing like the real thing.