Help choosing a long setback seat post?

Hello everyone,

I've recently purchased a retro road bike and have made a few upgrades, including a Brooks B17 S saddle. I've noticed while riding that the saddle position is a bit off and should be setback 1-2cm. Trouble is, my current seatpost already has a decent setback. I've measured it at roughly 26mm. It's an old Shimano ultegra that looks very close to this :

VeloBase.com - Component: Shimano SP-6400-A 600 Ultegra

I have an opportunity to acquire a VO grand cru long setback seatpost for a good price. It seems the difference in setback is only 5mm though, compared with my current Shimano. Is this enough to warrant a purchase? Does anyone have experience switching between the two? I'm also curious to know if the part that attaches to the rails can make a difference in position, specifically with a Brooks.

Thanks!