[Zanshin]

I've noticed a significant difference in my boat's performance, my starboard tack is about 1.5 knots slower than my port tack. I have a large water tank on my starboard side and thought that this might be a factor. Yesterday we had some very stong conditions and my destination was about 20 miles directly upwind, with the waves and current head-on as well. I had 2 reefs on both sails (25+ knots) and actually measured performance for the first time.

I would sheet in the genoa identically on both tacks, pulling the sheets back to the aft spreaders. I had adjusted the main until I had only about 2-6 degrees of weather helm on both tacks. This gave me 8.2 knots on starboard tack but only 6.5 or so on port tack. I don't have an inclinometer, but I was well heeled over and it seemed to be about the same angle, and I kept the angle to the wind identical on port and starboard tack as well.
I couldn't more than 7kn on port tack without letting out a reef. I'd emptied the starboard watertank completely, so I think that my boat is evenly statically balanced. The rigging looks straight and tensions are similar on both sides as well as fore-and-aft.

Is this rather large difference normal or acceptable? Is a rigger likely to find some way to balance this out? Could there have been some prior damage (i.e. grounding) which might have caused a slight deformation of the keel that can be causing this?