Rocksim has two potential issues.
First is that it tends to under estimate weight. Some of the materials are wrong, like the PML G10 fin material. Some of the problem is that we usually omit the glue and paint which can be really significant.
Second, we tend to over estimate the quality of our finish and hence under estimate the drag coefficient. 0.6 is on the low side and 0.75 is maybe a bit high. Try overriding CD with those values and see how the results compare to what you got letting Rocksim estimate CD. If you really want to go fast, eliminate the launch lug and use a tower or use a pop lug.
Weight and drag are your enemies. To keep fin weight down, skip the solid G10 and use a balsa core with fiberglass skins. It's not too hard to make. The fin cores could be something like 1/8" balsa for this size and engine. Both leading and trailing edges should be sharp, and I mean paper thin. Get some mylar from a drafting supply house and wax it with a couple coats of paste wax, buffing each coat. Lay on some 2oz fiberglass cloth and roll in some laminating resin (thin epoxy, not like what you get in most hobby shops) with a small paint roller. You want the cloth wet through and through but no puddles. You need two of these and each should be big enough for all 3 fins. Now lay the fin cores on one of the skin sheets and plop the other on top. You biggest challenge is now to squeeze the skins onto the cores. I use a vacuum bagging system. If you have one of those home vacuum food sealers, it should work, but you may not want to use if for food afterwards.
You could also put the layup between some stiff foam and pile books on top. Make the fins longer than needed and cut to length after glassing.
To laminate fiberglass (or carbon fiber) to a cardboard body tube, apply one or two layers, rolling laminating resin on with the small paint roller. Once done, wrap some peel ply around it. This is typically dacron fabric treated with a release agent so it will peel off the glassed tube. Now the trick! You need a long "airship" balloon. Blow it up, but don't tie it off. Start forcing it onto one end of the body tube, letting air out as you go. With practice and a long enough balloon, you can get it all the way onto the body tube before running out of balloon. I suggest practicing this before mixing your epoxy on a bare tube. It can be pretty frustrating.
If you doubt that this will hold up to a G80, a Der Red Max built just this way survived a G80 at LDRS. It did not survive getting stepped on during recovery, that broke one of the fins.
For max performance, you are going to want to use a body tube the saem size as the engine and surface-mount the fins. Glue on with epoxy as usual and then apply an epoxy fillet. I use the Locktite metal-concrete epoxy from Home Depot. Shape the fillet with a craft stick to get a 1/8" radius fillet. Now cut a strip of 2oz glass cloth about 1/2" wide and apply over the fillet, lapping onto the fin and body tube at each joint. Another approach is to make the reinforcing strip wide enough to go across the body tube between two fins and 1/4" up each fin. I think it's easier to handle the narrow strips.
Hope this gives you some ideas.
