using front Coaxial Speakers Alpine 4inch S2-S40 and rear speaker alpine S2-S65 also Coaxial Speakers.
If i add DSP such as helix, does it bring significant improvement to my sound???

btw the car is myvi lagi best.

what do you expect from Helix DSP? When you say improvement what kind of improvement you would be pleased?
DSP allows EQ
DSP may allows timing adjustment

EQ may give u effect that you may boost certain frequency that please your ear
Timing adjustment may improve ur soundstage if you already have proper speaker positioning.

the answer maybe yes and maybe no.. depends on what kind of improvement you are looking

front and rear speaker. got kena phase cancellation or not.
add dsp give a lot adjustment (headache pun) to u fine tune.

and fine tune means only specific song effect nice.
wont be cover every type of music

If you really wanna do.. can..
I would custom the front ensure 6.5 inch can be put at the door.. it's costly affair but worth it
4 inch have their limitations..

active or passive setup?