I often see comments about as flexibility improves. I started out with a 15 degree stem and as my flexibility improved I went down to a 10 degree, also 10mm longer. Now I'm thinking I'm ready to go lower again with also longer again.

Having some knee soreness I went forward with my seat which seems to have helped. With this I think I need to recover some of the distance I lost from seat to bars by going with a longer stem. Since I'm buying a need stem the question is should I stay with a 10 degree of drop to the more common 6 degree. Going from my now 120 x 10 degree to 130 x 6 degree will move my bar forward about 11mm and lower them about 7mm (from a vertical and horizontal plane). If I stayed with a 10 degree at 130mm it would move the bars forward about 9.6mm and raise the bars about 1.5mm.

I feel confident I have gained flexibility and that I could easily ride the lower position but is there any advantage or disadvantage to going lower?

Like most answers here in the 41, it depends. With your increased flexibility, what's your objective in a longer and/or lowered stem? Speed, comfort, endurance, power, etc?

Going to a longer stem to improve my position on the bike. I've had my seat too far back, with moving the seat forward I feel cramped so I'm going to go to a longer stem. I'm expecting it to be more comfortable, giving me more speed, endurance and power.