Let us assume my router is able to deliver at that value.
450mbit is not the actual speed. You will never see that speed when transferring real data.
Especially with off the shelf Access Points like the one you are using.
In WiFi networks those numbers are the theoretical maximum speed which is never the actual speed that you will get (there are numerous factors that will prevent you from ever reaching that theoretical maximum).
In practice you will get approximately half the advertized speed.
So in 802.11b 11mbit is roughly ~6mbits of real throughput.
In 802.11g 54mbit is roughly ~20mbit of real throughput.
In 802.11a 54mbit is roughly ~20mbit of real throughput.
Now in 802.11n it's a bit more complicated due to dual or triple chains and due to 20MHz or 40MHz channel width.
So in a single chain 20MHz channel 802.11n wifi you will get roughly 40-50mbit of real throughput.
With 40MHz channel width you will get roughly 70-80mbit of real throughput.
With dual or triple chain and 20MHz or 40MHz channel width you may get twice or triple the throughput, but as the rates go up, it becomes more difficult to sustain high speeds. The slightest interference will drop the rates and your real throughput.
In your case the wifi access point you are using has triple chain support, but you have to keep in mind that in order to achieve that kind of speeds your clients must also have triple chain wireless adapters.
Now, other than what speeds are actually attainable when you have multiple clients on an Access Point, it becomes even more difficult to provide guaranteed speeds. Especially if the clients are on the move and not on fixed points (such as in outdoor long range wireless links).
Also you have to keep in mind that in Point to MultiPoint setups such as yours, you are susceptible to the Hidden Node problem.
https://en.wikipedia.org/wiki/Hidden_node_problem
Hidden nodes in a wireless network refer to nodes that are out of
range of other nodes or a collection of nodes. Take a physical star
topology with an access point with many nodes surrounding it in a
circular fashion: Each node is within communication range of the AP,
but the nodes cannot communicate with each other, as they do not have
a physical connection to each other. In a wireless network, it is
likely that the node at the far edge of the access point's range,
which is known as A, can see the access point, but it is unlikely that
the same node can see a node on the opposite end of the access point's
range, C. These nodes are known as hidden. The problem is when nodes A
and C start to send packets simultaneously to the access point B.
Because the nodes A and C are out of range of each other and so cannot
detect a collision while transmitting, Carrier sense multiple access
with collision detection (CSMA/CD) does not work, and collisions
occur, which then corrupt the data received by the access point. To
overcome the hidden node problem, RTS/CTS handshaking
(IEEE_802.11_RTS/CTS) is implemented in conjunction with the Carrier
sense multiple access with collision avoidance (CSMA/CA) scheme.
There are some solutions to this that work quite well, but use proprietary protocols (such as Mikrotik's NV2).
Now, regarding your question about more powerful routers, I assume you mean routers with higher power output on the wifi.
This is not a solution IMHO. Wifi is a bi-directional communication. It's not like TV or Radio where you put an amplifier and more people can tune in.
If you have a powerful transmission your clients will be able to 'listen' to your AP better, but the signal from your clients to your AP will not be higher.
So that will not do you much good. And will cause more noise in your band (especially when you are using it indoors).
A general rule of thumb is to use larger antennas in order to have better gain. But that's not feasible with smartphones etc.
So to answer your question, what you need to do is use multiple access points in different frequencies (channels) and different locations each, so that the clients will use the one that has the best signal.
By using the APs in different locations depending on the topography of the area you want to cover you may want to use different antennas than the stock 2dB antennas that most off-the-shelf routers come with.
A panel antenna may be a good solution, but, again, it depends on your topography.
In general it's quite difficult to give you advice on what equipment to use when it comes to WiFi.
Since WiFi involves the science of RF it's quite different than wired networks and what we've been used to.
My advice would be to hire someone with experience on the subject to help you design the network properly.
That said, personally I would never provide any kind of SLA for Wifi networks. You simply cannot guarantee anything on an unlicensed band where anyone can use it (and therefore cause interference).
Also with the AP you are using I think you don't have any real control of the wifi part.
I would personally buy a Mikrotik Router (Routerboard) which would allow me to have extended control of everything.
Also I would definitely avoid using 2.4GHz. That band is filled with noise (especially in cities). It's pretty much an unusable band nowadays IMHO.