In response to why mod the J/24 rig rather than just sail with the blade.
1. The boat sails much better than with a blade in light winds. The J/24 with a standard blade is noticeably sluggish in light winds. With a full range jib about 10" longer on the luff than the blade, the boat sails about the same speed as with a Genoa in light to medium (up to 8). Although it is probably a little slower than with Genoa, we seem to beat the same boats by about the same amounts as before -they would kill us with a blade.
2. The boat sails better than with a blade in high Genoa range winds (8 to 20). The full length top batten can be twisted off to allow light crews to go faster than they could with the Genoa or a blade. The longer luff provides more power when it can be used. A full weight crew with a Genoa would probably still be faster.
3. The boat is nicer to sail. The boom is higher, decreasing head knocks and improving visibility below it. The primary winches are moved to the cabin top clearing a lot of space on the main deck and reducing bruises. The deck is cleaner because the jib clew position allows sheets to cross the boat just behind the mast.
4. Tacks are easier. The jib allows much faster and easier trimming than a Genoa in tacks. The cabin-top winch position allows easier tacks than even the blade with standard winch position.
5. The one-time cost is reasonable may be a cost savings in a year or two. A non-standard jib costs a little less than a class blade (~$1600 vs $1660), noticeably less than a Genoa (~2060-2380) and is used every race. After about 20 races, it still looks new. A new "A" Genoa after 20 races is usually showing enough wear to be demoted to "B" status. The mast can be raised 3" and the wires extended for less than $100 in materials.
6. Conversion back to standard J/24 can be done by a crew to do in less than an hour. Harken winches with pedestals can be moved in about 6 minutes each. The mast can be lifted a foot or so, the extension removed, and then lowered on to the step. The longest part is loosening and retightening the turnbuckles.