For applicants below 25.5 years old, even though you are 'chopped' (sorry to be using this umpleasant word) from the RSAF, it doesn't mean that you will be accepted by SIA. SIA's HR still has to check with RSAF to confirm that you have been 'released' before they will consider your application. Some guys, even though they have been chopped by RSAF, are not 'released' before they turn 25.5 years old.
Regarding visual acuity. The worst that will be accepted is 6/60. Put it simply, if you are 6/60 (without optical aid) and can be corrected to 6/6, you've met the requirement. But if your visual acuity is worse than 6/60, you are NOT considered to have satisfied the requirement even if you go for LASIK to get it corrected to 6/6. So the magic number is 6/60. Why do LASIK then? Maybe less restriction on your license to carry a spare pair of glasses? I can't think of any good reason really.
Hope this helps.
9V