Slats would be interesting and might help. Messerschmitt was very good at slats to the point that the first XP-86 Sabres had slat hardware salvaged from the Me 262.
First two notes:
1 When slats deploy they do not increase the coefficient of lift. What they do is allow the wing to increase the angle of attack by about 55%-60% (from 15 degrees to about 22 degrees) before stalling thereby allowing increased coefficients of lift which are essentially linearly proportional to the angle of attack. There is no heavy 'snatching' when the slats deploy. There is a slight moment change that can pitch the nose a little forward (a good thing) ut no immediate increase in lift. The downside of this 55% increased lift is that the L/D ratio deteriorates about 55% compared to getting the lift from a larger heavier wing.
2 One RAE report on captured Me 109E talk of uneven slat deployment 'embarrassing' the Me 109E. I think this is a case of pilots being unfamiliar. Nevertheless the Me 109F had a completely revised slat mechanism using tracks that deployed evenly and smoothly.
Now, as to the nature of wing stall characteristics depending on planform:
1 Rectangular planform has a favourable stall progression. The stall begins at the wing roots and progresses out toward the ailerons.
2 Trapezoidal planform (eg Me 109) has an unfavourable progression. The stall begins at the win tips thereby making the ailerons ineffective. Hence the Slats.
3 Elliptical planform (eg Spitfire) has an even stall progression where both the tips and roots stall simultaneously. The spitfire had thinner wing at the tips and these stalled earlier so the stall progression would have been unfavourable but for wing twist.
The cure this problem the spitfire had about 2.5 degree of washout twist from the wing outboard of the undercarriage attachment. This gave a nice progression of stall that the pilot could sense.
There is also the effect of propeller wash to consider as the prop slipstream means the inner wing is not necessarily at the stall angle as the angle of attack would suggest.
One problem the Spitfire would have is that the structure consists of a main spar at about 1/4 chord (where the lift is) with a thick leading edge skin to create a D structure that was very efficient and gave a lot of space in the leading edges. The secondary spar near the trailing edge didn't carry much load and it seems was there to provide torsional rigidity so that the ailerons didnt twist the wings. Putting big holes in the leading edge would require some engineering. There was some work in getting the Hispano to fit.
However slats on a Seafire, perhaps with reduced washout, would
1 Considerably increase angle of attack before a stall.
2 However this would require a higher angle of attack thereby effecting visibility on a carrier approach and even requiring a taller undercarriage. Rather than using it to lower approach speed the slats would best be used to create a much greater gap between approach and stall speed adding greatly to safety.
3 The spitfire had good spin/stall characteristics but it could still flip a wing. The slats would completely eliminate that.
As far as I can tell the Seafires needed a more robust undercarriage, shock absorbers that can absorb a higher sink rate yet not bounce the aircraft off the deck and probably a taller tail to improve low speed handling.
The slats would add some weight and drag though by reducing washout could reduce drag a little and even increase the Spitfire incredible critical mach. The increased lift at higher alpha would come at the expense of increased drag from a reduced L/D ratio so the turning circle would reduce it wouldn't be as much as expected and would slow the aircraft unless more power could compensate.
Adding some kind of fowler flap or double slotted flap rather than just split flaps would be better as these add lift without requiring a higher angle of attack. Both slats and fowler flaps would be even better. The slotted flaps or fowler flaps allowing a reduction in approach speed and the slats adding safety margin.
Roughly a fully slated wing had 50% more CLmax which that would lead to an inverse square root reduction in stall speed. 1/sqrt(1.5) so 82% i.e. 18% reduction in stall speed. Ultimately you would probably adjust other factors as well. A half slated wing can get rid of 2 degrees of washout which if the stall was at 16 degrees would be roughly 1/2 x 2/16 ie 1/16th increase in lift or about 6.25% more if you consider that the wing would still work with the root stalled, and the the stall would tend to develop at the slat junction and travel ineard and propwash would keep the alpha low less if you consider the taper reducing area on the outboard sections it may be a lot more. I think its probably more than this.
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