When you say the first 2 forms of Sui Lum Tao I am assuming that you are meaning the first 2 sections??
Sui Lum Tao has much to teach the WC practioner, it is basically where yu learn the ABC's of the entire system. It is true it does teach the main offensive and defensive moves, but this is only the begining. My Sifu always says to me that Sui Lum Tao is like like a buried treasure that after much time doing it, it will start to yield its treasure to you. Some of the fundamental principals that it teaches (and notice I said "some")
1. Economy of motion
2. Centreline theory
3.Confined area theory
4. Simplicity
5. How to find and maintain your centre of gravity
6. Maintaining and using your natural structure
7. relaxation and proper use of the shoulder as opposed to your muscular for attack and defence.
8. coordination of arms and stance
9. developing and utilising your natural energy through concnetration and "mind force"
10 Fixed elbow theory
11. The four gates and defensive halves
These are just some of the theories that with the proper time, patience and proper guidence by a good Sifu/instructor, you will discover with your Sui Lum Tao practice. Whenever you find what appears to be a problem with you wing chun, you always have the Sui Lum Tao to instruct you and guide you. It is often a story of Yip Mans students whenever they had trouble with aspects of their Wing chun that Yip Man would simply instruct them to "Do more sui Lum Tao" this is what my sifu also tells me "do more sui Lum Tao"
The way it enhances your natural bodies energy is through a 3 phase stage
1. Learning to relax and perform the movements in a TOTALY relaxed manner, so that you energy flows freely through you
2. Using your mind to perform the movments with your centre of gravity as the base and your shoulder as the fulcrum to do so.
3. Learning to direct that mind force and use it within your natural structure.
Sui Lum Tao should be performed slowly and with great concentration, always baring in mind the centreline, economy of motion and the other principles as highlighted, it is of vital importance that the first section is performed extremly slowly as it helps to form a foundation of movement and tasting of the gravitational forces that are at play, as well as the importance that the opening hands and their principals play in you Wing Chun. Another reason for performing the first section slowly (as well as the last 2 sections) is so that you can train your stance (there is more to talk about there but I will leave that for another time)
At first there will be the feeling ie muscular body feeling, in terms of movment this will dissipate over time and will start to gradually be replaced by a "mind" feeling which is not something I can encapsualte into mere words, it is crucial that you shoulder be totally relaxed at all times, so as to allow the strength of your stance support your hands.
In my opinion, dont be lazy like I was and rip yourself of, do Sui Lum Tao whenever you have the opertunity to do it, whenever you you find yourself with a spare 20, 30, 40 Minutes (based on how strong your stance and patience level is) but make sure when you practice it you do 2 things:
1. Do it exactly as your Sifu has shown you (there is a good reason for this that will become apparant later) ie DONT ADD ANYTHING TO YOUR SUI LUM TAO
2. Practice for perfection, try to do the moves as perfectly as possible with that sense of the principles (as stated before)
I will add one last thing.
With Sui Lum Tao it at first glance does not appear to be mush or even to teach that much, have patience and perservere through and dont be in a hurry, it has much to teach you but you must be patient to learn its lessons. I would advise any one starting Wing chun (IMOP) dont even worry about Chum Kiu or Bui Jee, they are simply extensions of lessons that are taugh in Sui Lum Tao. It really is the foundation, and the stronger the foundation the stronger your wing chun, infact I would quite easily say that someone who has studied and practiced Sui Lum tao indepth would quite easily beat someone who knows the entire system but has rushed through it.
I hop e this helps, if you have other questions I will be happy to answer them for you.