I wouldn't worry about losing speed when using an extension tube with the 165. I have a similar situation with my Bronica ETR/S cameras. If I use an E28 tube on one of my 150mm lenses I can get closer for portraits but this amount of extension is almost too much so I have to set the lens almost to the infinity mark to even things out. The net amount I have extended the lens by is less than 28mm. I finally found a 105mm f/3.5 Zenzanon which gets closer and is long enough compared to the standard 75mm lens to give a pleasing perspective. Adding an extension tube doesn't turn an f/2.8 lens into an f/4 lens. It simply reduces the amount of light hitting the film. If the f/2.8 lens had a long enough helicoid to get you as close as you want to be, you would lose just as much light. Most 6X7 shooting is not done hand held. For the purpose of portraits I think the Mamiya RB67 is easier. The built-in bellows can get you closer than you would get with most other medium format lenses which have built-in helicoids. I do hand held shooting with my Bronica GS-1 but I only have the 100mm focal length. For portraits I think I'd like to have a 200 but the 200 is more expensive, heavier and slower than a 150.