Sorry, I made an assumption that Dilbert work is a flat contract (aka, no extra pay for extra hours).
If the extra hours are paid then indeed a 5% increase seems reasonable as it would really be 5% increase. Plus the 100% extra hours means a bigger paycheck.
So either he is being offered a fair deal, or he is getting highly screwed. It would depend on his contract. Interesting enough a flat contract would be ideal for Wally. As he is getting paid for not working.