Hi Eric B,
The 244 boom will work on the 8.5 m2 rig just fine. You only need an extra 10 cm for adjustments. But the larger Course boom would be a good upgrade.
Before we go any further, what size boards and sails are the other sailors at your lake using.
I based my suggestion for the 8.5 m2 rig purely on the size of the board and I'm not sure how/why you selected the Futura 141.
As far as the mast extension, just get the mid size 28 cm.
Putting too much mast base up inside you mast can cause mast breakage and interfere with the mast bend on some sails at the lower batten creating bottom batten rotation issues.
The device you are talking about ( as Coach suggests) can be hard on masts, has been know to "rip" the mast box out of some boards, and to me it's just terrifying. If you ever got a finger, toe, arm or leg caught on top of the wheel think of the forces 20 feet of mast could exert at about 3" from the pivot. Scary!
Also, when you get to waterstarting, the Deviator will be totally in the way.
I like the multi-density foam nose pad which you can remove once you get beyond the catapault stage.
Check with the other sailors at your lake, and make sure that the Futura 141/8.5 m2 rig is a good combination for the conditions there.
If not, you might consider a smaller board and rig.
Hope this helps,