Some background.
The Imax does perform very well on 10/11/12/15.
17 already shows that the antenna can be tuned, it is not optimal.
Forget 20 and yes I run the Imax with radials up to 4 x 5 meters and original GPK package....
Chosing the main radiator element length I worked my way down from 31 feet to now 25 feet, went as low as 20 feet to see what the effects were in SWR, radiation resistance and efficiency.

Normally we pick a length that is maximum 5/8 on the highest frequency because otherwise the vertical will has it's lobes there pointing up in the sky if we use a longer length.
That would be 6.6 meters length, or around 22 feet.
With that length the 40 meter suffered quite in efficiency ( still well under 1/4 wave there) so not very effective low rr so high losses.

Picking the 25 feet proved to be a lot better, SWR now elevated 5 feet over ground and 2 radials free of the ground was very satisfactory.
Running 200 watts carrier in the antenna for 10 minutes on each band didn't heat up the 1:4 unun, ( Ft240 - 43 core)

All other bands were almost perfect, worst is 1:1.3....
The stinger raises the 10/12 meter current point and lowers the lobs to a dx portion instead as looking up too much, no other effects on the other bands, see the stinger as a stub.
after instaslling the stinger tune it to just over the 10 meter band 30.5 to 31 MHz for best effect.
Check with beacons on your reception signal for maximum effect.

On my chimney I see this as the longest vertical without guying I can reliably get away with ( concrete chimney) and force 12+ gales we have here sometimes.
That was the reason to pick the supply of Titanex tubing I have around here, from 1 3/4 inch up to the smallest tube on top of 20 millimeter, wall thickness 1/10th of an inch.

The FT240 - 43 was chosen to have a reasonably high Mu and therefore enough high value, 7 MHz is almost 20.000 Ohms, 29 MHz is 82.000 Ohms.
Mu is 850.
The same cores are used as wideband chokes here good for 1500 watts ( single core) and Teflon 5 mm coax around them.
I'll run my definitive tests with 800 watts later in the weekend when the stinger is at it's place permanently.
I tested T225 - 2 cores, and found them lacking, they handle high power but they need 2 x 19 bifilar windings to be a bit effective for 40 meters, and then the capacitance starts to throw things off the rocks.
Too long windings around the core form capacity and will put things off the scale.
Max length of wire on the cores is 10% of the highest frequency in use ( 10 meter max wire length 1 meter).
Put tape on the FT 240 - 43 cores they conduct,so isolate them and use Teflon wire to make the unun.

The stinger idea was from here: http://www.eham.net/articles/29052, found late yesterday evening and made today.
Read that article including the piccies diagrams lobes for each band etc.
I already decided to 25 feet max radiator length so the stinger was optional.