It is worth noting that SRS is a deferred taxation scheme, as other people have already mentioned. More importantly, only **50% of withdrawals are taxable**. That's actually the most beneficial part of the scheme, more so than the deferment. That said, any withdrawal from the SRS is taxed even upon death (like if you end up with several millions in the SRS that you wish to pass on, those millions might be taxed at 12%, see below for why)
Anytime you need to move a huge lump sum of money out of the SRS (estate, large investment etc.), the highest tax rate would be **24% x 0.5 = 12 %**(highest tax rate x 50% of withdrawal is taxable). So if your current tax bracket is more than 12%, you stand to gain even if it is just a little (There are some assumptions I am making in this statement). If your SRS account is growing much faster than inflation, then you might end up paying more taxes on those gains, somewhat similar to a **capital gains tax**. At the extreme, you might end up in a situation where you save 24% on 80k annually, but pay 12% on millions when withdrawing.
My heuristics when thinking about the SRS are:
1) Putting in only a portion of your disposable income if your tax bracket is high so that the savings are worth the liquidity hit and deferred tax hit.
2) Making sure SRS has enough at retirement to supplement monthly expenses in the future, **but no more**. SRS is most effective when you are never withdrawing a huge sum of money, especially for estate purposes (i.e. on death).
3) Because **only 50% of withdrawal is taxable**, the highest tax rate is capped at 12%. Effective tax rate will be much lower. That means for most people, even if they don't min-max their SRS, they won't lose out that badly. For example, if you plan on withdrawing a relatively large amount of $240,000 in a single year ($20,000 a month at today's rate!), your effective tax rate would be \~3.3%. (7950 tax for the 120,000 taxable income. Divide 7950 by 240,000.) If you withdrew 2 million that year, the effect tax rate would be \~10.0%. For most people, the effective tax rate will be lower than 3.3%.
So based on your bracket, suppose you are saving 11.5% on 40,000 (4600), in 20 years at 5% compounding, you would get
1) Non-SRS $35,400 compounded over 20 years \~$94,000
2) SRS $40000 compounded over 20 years plus 3.3% tax on withdrawal (based on my assumptions above) \~$102,500
So putting it in SRS today earned you about \~$8500, or \~9% more, after 20 years. If your current tax bracket is higher, if the money compounds at higher than 5% annually, if you plan to withdraw less than my very generous assumption at retirement, if you are only retiring after 30 or 40 years, the difference will be even bigger.
(I rounded some numbers for easier interpretation. Also, these calculations make some assumptions, like making huge withdrawals during retirement. If my math is wrong, please let me know so I don't spread erroneous information!)