I believe there have been upgrades to the valve body for the 19+ models. However, I still read quite a few threads on here and FB relating to valve body issues (accumulator plates/solenoid packs).
Tuning the 68s has its pros and cons. With increased line pressure you will see less clutch wear but increased line pressure can potentially lead to accumulator plate issues and cross bleeding as well. I much prefer how a tuned 68 drives compared to stock, if you are hard on it you will have issues but if you drive responsible your 68 will live a long life whether its tuned or not.
The thermal bypasses in my opinion only belong on full out race trucks. The transmission is designed to operate at a specific temperature and a thermal bypass delete will not allow your transmission to reach operating temperatures. To my knowledges the 68s run cool in stock configuration. I generally see 160-170 and compared to my previous Fords I am extremely happy, if it aint broke don't fix it!