I've been very pleased with the performance gains from 6.2 to 6.3. I had a very frank discussion about Kaseya with my Service Desk Engineers just last night, and noted that they unanimously agreed that:
> The interface is "zippier"
> The VNC-based Remote Control connects much faster
> Live Connect is "solid"
That said, how something "feels" is often subjective, so to quantify it, here's what we've got (bearing in mind that we've sized this according to the System Requirements listed here: http://help.kaseya.com/WebHelp/EN/VSA/6030000/Reqs/K2-System-Requirements63.htm)
Agent Supported: 4,500 and counting
Modules In Use: See below...
Both Frontend & Database Servers:
> Windows Server 2012
> Virtualized using VMWare ESXi 5.1
> Storage is on a NetApp 3210, communicating via fiber & NFS
Frontend Server
> 12 GB RAM
> 4 Cores (1 socket, 4 cores/socket)
Database Server
> 24 GB RAM
> 16 Cores (4 sockets, 4 cores/socket)
> Database, TempDB, OS and SQL backups on separate volumes
I think one of the big keys here is the reservations that we have in place with our virtual infrastructure. I watched and took many notes on the Performance Tech Jams (Part 1: http://community.kaseya.com/resources/m/techjams/72733.aspx, Part 2: http://community.kaseya.com/resources/m/techjams/79493.aspx) as well as the Scaling Kaseya Servers Tech Jam (http://community.kaseya.com/resources/m/techjams/74666.aspx), and worked extensively with our Project Engineers to make sure our infrastructure could handle the requirements of this system (which is obviously business critical for us).
I should also note that the "Modules In Use" figure I'd mentioned above doesn't do justice to the depth at which I'm utilizing these modules... our ScriptThenElse table is nothing to shake a stick at, and any managed machine might have as many as 20-25 different Policies applied to it--not to mention 3-6 Patch Policies applied to each, and over 50 Views used for Policy Management alone... so we're definitely giving the system a workout. 
Anyway, on to the pretty pictures... these are from today (Friday) when we've had a "full house" of Field Service Techs, Service Desk Engineers, Project Engineers, etc. and all of our clients were "in business." Also of note: many of our clients' workstations patched this morning as well.
Total CPU Utilization On Database Server:
Disk Latency On Database Server:
Memory Utilization On Database Server:
Total CPU Utilization On Application Server:
Disk Latency On Application Server:
Memory Utilization On Application Server:
So, hope that helps offer a baseline for what we're seeing in the "real world" with around 4,500 Agents.
There are many other threads talking about the benefits of 6.3 (I love the new multi-tabbed Agent Procedure editor 
), but as this is a performance-related thread, I'll just leave it here by saying that we're quite pleased with the results of the performance enhancements that the Kaseya Devs have been working on... we definitely feel it. Keep it up!