RealTerm: v2.0.0.43
CP210x VCP driver: 5.1.0.0

IOCTL_SERIAL_SET_QUEUE_SIZE - (maybe issued by SetupComm())
InSize: 65520 (0xFFF0)
OutSize: 16384 (0x4000)
return code: STATUS_UNSUCCESSFUL (0xC0000001L)

The usual coding manner to change the default IN/OUT queue size is,
1) call GetCommProperties()
Then the maximum queue size is returned in
dwMaxTxQueue and dwMaxRxQueue fields of COMMPROP structure.
These queue size are
a) Hard-coded in the device driver
OR
b) setup on the registry by the INF file

2) set the queue size by SetupComm()
The queue size is determined according to dwMaxTxQueue and dwMaxRxQueue.

RealTerm seems not to follow this convention.

Unfortunately, we cannot access to the source code of RealTerm.
Report it to the support forum for RealTerm.

[References]
RealTerm: http://realterm.sourceforge.net/
PortMon: http://technet.microsoft.com/en-us/sysinternals/bb896644.aspx
IOCTL_SERIAL_SET_QUEUE_SIZE: http://msdn2.microsoft.com/en-us/library/ms800971.aspx
SetupComm: http://msdn2.microsoft.com/en-us/library/aa911370.aspx
GetCommProperties: http://msdn2.microsoft.com/en-us/library/aa911509.aspx

It returns these result in COMMPROP structure, as the default setting.
dwMaxTxQueue = 0
dwMaxRxQueue = 0
dwCurrentTxQueue = 0
dwCurrentRxQueue = 512

Then, report it to SiLabs also.

"Realterm installation does come with the Delphi 7 based source codes (you need to choose this option during the installation)."

Ah, thanks.
I wasn't aware of it.

"But I am not so sure if this helps to identify the problem as it seems to use some external components."

Ya, RealTerm uses this component. I found it from the source code of RealTerm.

"TurboPower Async Professional"
http://sourceforge.net/projects/tpapro/

As you said, the problem lies in the implementation of the component.
The unit AdPort (AdPort.pas) should provide some interface to return dwMaxTxQueue and dwMaxRxQueue from GetCommProperties() (Win serial API). To implement this interface, AwWin32.pas should have a wrapper of GetCommProperties().

Anyway, if SiLabs new VCP driver just succeeds IOCTL_SERIAL_SET_QUEUE_SIZE, there is no problem even without any modification on the RealTerm side

I sent a bug report to SiLabs, as follows.

Bug report on CP210x VCP driver

Driver version: v5.1.0.0
These bugs are found in TXQueue and RXQueue handling.

a) IOCTL_SERIAL_SET_QUEUE_SIZE
fails in this input parameters, though v4.40 succeeds.
InSize: 65520 (0xFFF0)
OutSize: 16384 (0x4000)
return code: STATUS_UNSUCCESSFUL (0xC0000001L)

b) IOCTL_SERIAL_GET_PROPERTIES
returns these false result in COMMPROP structure, as the default setting.
dwMaxTxQueue = 0
dwMaxRxQueue = 0
dwCurrentTxQueue = 0
dwCurrentRxQueue = 512