This point interests me, because I keep hearing comments about how "Malaysia's secondary school syllabus has been watered down", and I am actually wondering how true that statement really is.

I have been exposed to both the SPM and Cambridge GCE syllabuses before, having done my secondary school both in Malaysia and Singapore in the early-1990's:
1. I took Malaysia's Matematik Tambahan and Singapore's Additional Mathematics textbooks and past year papers (i.e. SPM vs. GCE 'O' Levels), and laid them side-by-side. The syllabus was almost exactly the same.
2. SPM Biology required students to study most of the bones in the human body. Singapore's G.C.E 'O' Level Biology covered only four (4) bones on the arm.
(I am aware that G.C.E. and IGCSE are not the same thing, but I am guessing that they should be at least comparable in standard.)

Does anyone have any concrete data and information to support or debunk the claim that Malaysia's secondary school syllabus has really been watered down, at least for Mathematics and Sciences? Note that I am referring to the syllabus content and level of difficulty, not the marking scheme (which is another big topic for debate) or the quality of teaching (which is highly-dependent on individual schools and teachers).
I agree with the first part about the optimum combination being SJK©+SMJK©. When I was doing my senior high school in Singapore, one of my schoolmates was ex-Catholic High School PJ. He was winning Chinese essay awards, trouncing his Singaporean classmates, and his English was strong enough to keep up with the Singapore standards.
On that basis, I personally believe that more SMJK©'s are required to meet the high demand (I saw two newspaper articles about Catholic High School appealing for a branch school to be established, as it is the only SMJK© in the whole of Petaling Jaya, and has to turn away a large number of applicants).

Per my above point, if it can be established that the national secondary school syllabus - again, purely in terms of content and level of difficulty - is not inferior to IGCSE and its equivalents - then to me, we can and should start by leveraging from the standard of teaching and execution by the good SMJK©'s. Taking a leaf from my senior high school experience in Singapore in the 1990's: Regardless of school, the syllabus is the same, and not so different from Malaysia (like I said, the same Additional Mathematics syllabus was taught on both sides of the Causeway). The reason why the standards in Singapore were much more consistent nationally, and generally higher than that of Malaysia, lay in the quality of execution. The teachers were dedicated and focussed. They spurred and inspired the students to try harder.

Outside of classes, extra-curricular activities were better-emphasised and executed. Let me put it this way: It's not as if the ECAs that I experienced in Singapore were far different from Malaysia. They have Scouts and Cadets in Singapore, but hey, so do we in Malaysia! But in Singapore, they really make uniformed groups a part of student lifestyle and pride.