power difference between 10mm & 11mm head

[the caveman]

I already have modified my IP for my 1.9 na transporter. I already had the green timing spring , played with the governor and replaced the 9mm head with a 10mm. I had bought an 11mm but didn't have the time to match it to the idi pump. How much difference can i really expect if i get an 11mm put on. how much more if i have Giles or TinTin build it up? I'm not unhappy with what i have ,but could use enough extra torque so i don't have to use 1 st gear [5 speed vanagons have 1 st on a dogleg,  going from 1st to 2nd isn't great ]. As it is 2nd is okay on a level surface and i slip the clutch, but i don't have quite enough torque for all the time. As i stands i'm pretty sure i'm close to the same HP and torque as a stock 1.6 TD