Hi,
You can try patching the <VisualGDB>\MSBuild\embedded.targets file (see the ChangeExtension part below):
<Target Name="GenerateHexFile" DependsOnTargets="Link" Condition="'$(GenerateHexFile)' == 'true' and '$(GNUTargetType)' == 'Application'">
<GenericGNUTool Sources = "$(TargetPath)"
ToolPath="$(GNUToolchainBinaryDirectory)"
ToolchainPrefix="$(GNUToolchainPrefix)"
ExtraPath="$(GNUToolchainExtraPath)"
VisualGDBSettingsFile="$(VisualGDBSettingsFile)"
ToolExeNameBase="objcopy"
ResponseFileTag="mkihex"
RepeatForEachSource="true"
PrimaryOutput="$([System.IO.Path]::ChangeExtension($(TargetPath),'ihex'))"
FastUpToDateCheckDatabaseFile="$(FastUpToDateCheckDatabaseFile)"
RemoteBuildMakefile="$(RemoteBuildMakefile)"
CommandFormat="-O ihex $< $@"/>
</Target>