Sorry for the long delaay in my reply here. I have been working aboard the ships and traveling to demos pretty much non-stop since you posted your request. I also did not have internet access over the weekend.
As far as the "differences" between your 85 cm wide F-155 and the 2003 F-147, I think you are going to find the F-155 was better with small sails like your 8.0 m2 Retro.
The F-147 will definitely go upwind higher and faster, probably go off the wind faster, but with only your 8.0 m2 Retro, I'm not sure you will have the power you need to take full advantage of the F-147 with a 70 cm fin.
If you drop down in fin size to say 62-65 cm, you will lose a little upwind performance and early planing, but with only an 8.0 m2 rig that may be the best compromise.
If you aren't racing, I wonder if the F-Type or 2004 Free Formula 138 might not be a better board for recreational sailing. These boards would be a definite upgrade from the F-155, but they do much better with smaller sails.
If you had a 9.5 m2 Retro, then the F-147 might be better.
As far as angles and speed, the F-147 or F-Type 148 will be significantly better than your F-155 with the 58 cm fin both upwind and really far off the wind due to the shorter plaing surfaces and greater width.
As to reaching speed, I think your F-155 may have a small advantage, unless you get a 9.5-11.0 m2 rig to go on the wider F-147 or F-Type 138/148.
Going downwind (recreationally) should not be really hard, just scary.
Hope this helps,