I shoot with a 1Ds2 and the 17-40, so I'm at a true 17mm. To be honest, that is too wide for a lot of rooms. If you are shooting with the 8-16, then you are at 12.8mm, which is really too wide. That may be contributing to the problem. Try zooming in to about 12mm and see if that doesn't make it easier to straighten things out. I know at 8mm there has got to be some barrel distortion going on too. That is just an assumption, but when you straighten those lines up you will see. Even my 17-40 will give me just a tab of barrel distortion at 17mm. Yeah, its a quick fix, but I'd rather just zoom in to 19mm and get rid of it in camera.
We have a guy from San Diego that posts quite a bit on that thread I shared. You may want to introduce yourself and see if there is an opportunity to meet up and talk shop. It is so much easier to have someone show you what to do than to try and figure it out through trial and error like I did. LOL Luckily, I am in a town where I have very little competition for good photos. That will not be the case in San Diego. You are going to have some very good photogs there, I'm sure of it.
I lost a lot of clients in the beginning because of the errors I made. Many of them came back once I got the hang of it, but quite a few did not. One in particular would order about 30-40 shoots a year. That one hurt! Get the basics down, straight lines (vertical), good lighting, good composition, and focus on the 'features'. This is where my background as a Certified Residential Appraiser really helps me. I can walk into any home in my market and I know the noteworthy features, I know the differences in the woods, the cabinets, the granite, etc. I make sure to showcase that in my photos. The buyers are out shopping for homes, it is your job to show they why this particular home is better than the others. I would suggest that you ask you client this very question 'Would you mind showing me all the noteworthy features of this home so I can make sure to photograph them?" You don't have to say 'noteworthy' it is a term I used often in appraisal reports. You can use 'selling points' or 'best features' as well. What this does is it immediately tells the client that you are there to help them sell this house. You are focused on capturing the best this house has to offer. It makes you an asset instead of an expense, and that will result in repeat business.