ip route 192.168.200.0 255.255.255.252 GigabitEthernet0/1
is a useless entry, the router knows better than any administrator what networks it is connected to.
In regards to STP, well that is a complete different topic. I suggest the standard L2 security features, at the very least enable Portfast and BPDU-Guard on all edge ports.
Well spotted. I forgot to delete that static route. I had it configured to check some differences however you are right, it is unnecessary.
In relation to portfast and BPDU-guard on all the access ports. We have some users connecting their small 4 or 8 port unamanged L2 switches to be able to connect more devices via a single port in the wall. Would it not create a loop when portfast is enabled? Also if BPDU-guard is enabled, this would mean a lot of work for me to either remove both settings from the specific interface and restart it if user connects their small l2 switch.
I am thinking about implementing EIGRP redistributed network so we don't have to play around with static routes and it is dynamic. This way, I can implement subnets directly from the catalyst 3750. What do you think about it? Instead of static, go dynamic.
I was also thinking about /30 network in between a router and a switch. What are the benefits of having /30 network in between, over normal sub-interfaces on a router and trunk instead of /30 network? I assume that the only traffic that would have to go up the link would be MPLS, WAN or another subnet on the same router. Is that correct? Local subnet would be switched on the switch without need to go to the router.
Neither Portfast nor BPDU-Guard is going to create a loop. In fact, BPDU-Guard is one of the best ways to detect and break a loop. You don't want to remove it from ports connected to unmanaged switches, quite the opposite in fact. There are alot of documents out there describing the technology and when and where it shoud be used.
If you feel comfortable with dynamic routing protocols, then go ahead with it. I'm usuallly a big fan of dynamic routing, because it makes managing big networks so much easier. If you should go ahead with EIGRP is another question. If you already do use OSPF as your posted config suggests I would go ahead with OSPF, otherwise you need to configure route redistribution from one protocol to another, which adds a lot of complexity and other disadvantages.
To your last question, what is the benefit of a layer-3 port over subinterfaces. In your small setup there's not much difference either way. However it is good practise to keep your layer-2 domain as small as possible. First, you spare a vlan id for future use, second and more important, you keep spanning tree out of it. Pure layer-3 connections usually have a much better convergence time than switchports that need to go through all spanning tree phases before passing traffic. When you start using dynamic routing protocols it gets even more important. Rule of thumb: Use Layer 2 in the access where it's needed, don't use it where it's not.