I would say the opposite : you do not need to increase the high speed of the sonic for lightest wind, you need a more powerful loop and more grunt at low speed, and longer pull duration, i.e. get longer lines.
Now about a kite with better low end than Sonic2 , here is my opinion:
- I tried or owed the following kites: conceptair Pulsion, Kitech FRS, Sonic 2, Speed4 Lotus, Pansh A15 and Aurora v2 , Elf J5 ; and I discussed a lot with friends seeking for low end with a Pulsion and having compared with Hyperlink, PL Aero, Soul
- a medium ratio kite will ease the waterstart and the keeping the kite in the air (Pulsion, Hyperlink, Soul, FRS, A15, Aurora, speed4), which are often the two more difficult steps in extreme light wind; e.g. this is the main reason why the Soul is better than the Sonic 2 in extrem light
- the lightest , the best ; sonic Fr was a hammer with a poor low end -> Sonic 2 is improved -> but R1V2 remains better (like PL Aero V1); Soul is improved vs Sonic2, but not the best; FRS is a bit heavy so less usable than Soul , Hyperlink standard is also a bit heavy ; Conceptair Pulsion beat them all and currently the winner on the weight and on its ability to stay in the air with or without working the kite (confirmed by all users I know and that were able to compare with Soul, Sonic2 , Aero, Hyperlink or FRS) ; FRS is less stable at the zenith and needs a bit of line tension/working compared to Pulsion. A15 is very stable ; Soul not tested on this point.
- another point is the agility and power during the loop for the waterstart; on this point FRS is worse than others with a lack of power during the loop (but excellent depower and agility even with slack lines, but not powerful) ; Soul is better but not the best, a little lack of agility unfortunatelly ; A15 has a (too) large loop but quite powerful; Conceptair Pulsion has a bit of delay between the pilot command and the loop, but when you know this, turns quick and smooth and gives you tons of power when crossing the window (so the best IMO for the waterstart and working the kite)
- relaunch : from my experiments I would not say one is really above others, they have pro and cons : Soul and FRS are very watertight so can stay in the water for a long time ; Souls relaunches with a bit less wind ; Hyperlink is in-between these two ; Pulsion is less watertight so more sensitive to pilot error during the relaunch or if the lulls duration exceeds 5 minutes ; BUT it relaunches easy and in less winds than all others in the firsts minutes (which maybe crutial).
- high end : FRS has an excelent depower vs Pulsion (actually its windragne is relocated in higher winds), Soul is in-between
- robustness : FRS and Soul seem very godd, Pulsion seems more fragile on intercells in case of crashes.
- drift (important for waves or transition in very light wind ) : 1- Pulsion, 2- A15 and Soul and HL, 3- FRS
Open to gentle comments of course !