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>